Fix: Remove extra quote in Slides sample docstring

[Mariatta]

There's an extra quote in the docstring, causing the code snippet to not be rendered with correct syntax highlighting.

If you go to this page, the snippet look like it's all commented out.
https://developers.google.com/slides/api/guides/create-slide#python

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes internal issue # 237935769

Has it been tested?

• Development testing done
• Unit or integration test implemented

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have performed a peer-reviewed with team member(s)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/