Rename client to email_client so that send examples work

[stevensnicole]

The naming of the client and email_client are different between the authentication examples at the top and the send examples at the bottom, without this change the full example fails.

[learn-build-service-prod[bot]]

Learn Build status updates of commit e6606d1:

:warning: Validation status: warnings

File	Status	Preview URL	Details
docs-ref-services/latest/communication-email-readme.md	:warning:Warning	View (azure-python)	Details

docs-ref-services/latest/communication-email-readme.md

• Line 9, Column 13: [Warning: ms-service-subservice-invalid - See documentation] Invalid value for 'ms.service': 'communication'.
• Line 2, Column 1: [Suggestion: description-missing - See documentation] Missing required attribute: 'description'.
• Line 5, Column 12: [Suggestion: ms-author-invalid - See documentation] Invalid value for 'ms.author', 'acsdevx-msft' is not a valid Microsoft alias.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel