docs: fix links to CONTRIBUTING.md

A few of the `CONTRIBUTING.md` link were invalid, either due to
referring to the `main` branch instead of `master`, or missing the
`/blob/` segment in the path.

Resolves: #592
Signed-off-by: JP-Ellis <[email protected]>

Author: JP-Ellis

.github/ISSUE_TEMPLATE/bug.yml

@@ -27,7 +27,7 @@ body:
     attributes:
       label: Have you read the Contributing Guidelines on issues?
       options:
-        - label: I have read the [Contributing Guidelines on issues](https://github.com/pact-foundation/pact-python/main/CONTRIBUTING.md#issues).
+        - label: I have read the [Contributing Guidelines on issues](https://github.com/pact-foundation/pact-python/blob/master/CONTRIBUTING.md#issues).
           required: true
 
   - type: checkboxes

.github/ISSUE_TEMPLATE/feature.yml

@@ -15,7 +15,7 @@ body:
     attributes:
       label: Have you read the Contributing Guidelines on issues?
       options:
-        - label: I have read the [Contributing Guidelines on issues](https://github.com/pact-foundation/pact-python/blob/main/CONTRIBUTING.md#issues).
+        - label: I have read the [Contributing Guidelines on issues](https://github.com/pact-foundation/pact-python/blob/master/CONTRIBUTING.md#issues).
           required: true
 
   - type: textarea

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1,13 @@
 <!--
 Thank you for sending the PR! We appreciate you spending the time to work on these changes.
-You can learn more about contributing to Pact-python here: https://github.com/pact-foundation/pact-python/blob/main/CONTRIBUTING.md
+You can learn more about contributing to Pact-python here: https://github.com/pact-foundation/pact-python/blob/master/CONTRIBUTING.md
 
 Happy contributing!
 -->
 
 ## :airplane: Pre-flight checklist
 
--   [ ] I have read the [Contributing Guidelines on pull requests](https://github.com/pact-foundation/pact-python/blob/main/CONTRIBUTING.md#pull-requests).
+-   [ ] I have read the [Contributing Guidelines on pull requests](https://github.com/pact-foundation/pact-python/blob/master/CONTRIBUTING.md#pull-requests).
 -   [ ] **If this is a code change**: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
 -   [ ] **If this is a new API or substantial change**: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.