fix(cli): outdated instructions in README of python init template (#1047)

Fixes issue [#9135](aws/aws-cdk#9135) by
updating the README to be consistent with [AWS
docs](https://docs.aws.amazon.com/cdk/v2/guide/work-with-cdk-python.html#work-with-cdk-python-dependencies)

Note that setup.py was removed from the python init-template in PR
[#17062](aws/aws-cdk#17062)

---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Co-authored-by: Gowthami Gudipati <gudipatg@amazon.com>

Author: gudipati

packages/aws-cdk/lib/init-templates/app/python/README.template.md

@@ -44,7 +44,7 @@ $ cdk synth
 ```
 
 To add additional dependencies, for example other CDK libraries, just add
-them to your `setup.py` file and rerun the `pip install -r requirements.txt`
+them to your `requirements.txt` file and rerun the `python -m pip install -r requirements.txt`
 command.
 
 ## Useful commands