Fix Typos

[taspeotis]

Changes

I spotted a typo in CoreEventSource. Turns out there were a few others. And then when I created this PR there was another typo in the PR template.

This change is not substantial, so I am omitting to complete CHANGELOG.md unless you would like me to.

There may be other typos left elsewhere; I only set out to fix TelemetlyClientFlush

Checklist

• I ran Unit Tests locally.

• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.