Fix code sample typos for Context Manager Examples in README.rst file

[Jastor11]

Description of Change

There are some apparent errors in the Context Manager Examples sample code displayed in the README.rst file

Assumptions

The replacement example is what is the intended usage for context managers. Feel free to correct any mistakes I made here.

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
• Detailed description of issue
• Alternative methods considered (if any)
• How issue is being resolved
• How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
• Detailed description of new feature
• Why needed
• Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[thehesiod]

ty!