fix: corrected header in python (#586)

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# Fixes #

Correcting application/json header in python

### Checklist
- [x] I acknowledge that all my contributions will be made under the
project's license
- [ ] Run `make test-docker`
- [ ] Verify affected language:
- [ ] Generate [twilio-go](https://github.com/twilio/twilio-go) from our
[OpenAPI specification](https://github.com/twilio/twilio-oai) using the
[build_twilio_go.py](./examples/build_twilio_go.py) using `python
examples/build_twilio_go.py path/to/twilio-oai/spec/yaml
path/to/twilio-go` and inspect the diff
    - [ ] Run `make test` in `twilio-go`
    - [ ] Create a pull request in `twilio-go`
    - [ ] Provide a link below to the pull request
- [ ] I have made a material change to the repo (functionality, testing,
spelling, grammar)
- [ ] I have read the [Contribution
Guidelines](https://github.com/twilio/twilio-oai-generator/blob/main/CONTRIBUTING.md)
and my PR follows them
- [ ] I have titled the PR appropriately
- [ ] I have updated my branch with the main branch
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added the necessary documentation about the functionality
in the appropriate .md file
- [ ] I have added inline documentation to the code I modified

If you have questions, please create a GitHub Issue in this repository.

Author: tiwarishubham635

src/main/resources/twilio-python/listOperations.handlebars

@@ -16,7 +16,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -42,7 +42,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -63,7 +63,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -84,7 +84,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -104,7 +104,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -122,7 +122,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -145,7 +145,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}
@@ -171,7 +171,7 @@
                 {{#headerParams}}'{{{baseName}}}': {{paramName}},
                 {{/headerParams}}'Content-Type': 'application/x-www-form-urlencoded'
             })
-        {{#bodyParams.0}}headers = {'Content-Type': 'application/json'}{{/bodyParams.0}}
+        {{#bodyParams.0}}headers['Content-Type'] = 'application/json'{{/bodyParams.0}}
         {{#queryParams.0}}params = values.of({
                 {{#queryParams}}'{{{baseName}}}': {{paramName}},
                 {{/queryParams}}