Update generating-a-json-web-token-jwt-for-a-github-app.md

[frjtrifork]

Why: Python JWT example script throws an error without cryptography module

In a fresh python installation the python script for getting a JWT needs both the 'PyJWT' and the 'cryptography' pip packages installed.

Can be verified using a fresh docker python:3-alpine image:

docker run -v $(pwd):/app -w /app python:3-alpine \
  sh -c 'pip install PyJWT; python githubJwt.py ghapp.private-key.pem CLIENT_ID'

The command above will throw an error:
NotImplementedError: Algorithm 'RS256' could not be found. Do you have cryptography installed?

If you add the cryptography package to pip install, it works:

docker run -v $(pwd):/app -w /app python:3-alpine \
  sh -c 'pip install PyJWT cryptography; python githubJwt.py ghapp.private-key.pem CLIENT_ID'

Closes: #36104

What's being changed (if available, include any code snippets, screenshots, or gifs):

The pip install note above the Python example script is being updated to include both the 'PyJWT' and the 'cryptography' packages.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the preview environment.

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[github-actions]

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Source	Preview	Production	What Changed
apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[felixArbucias]

LGTM