Fix typos in MainThread.cs

[ivosnoza]

Description of Change

Fix typos aysnc -> async in MainThread.cs

Bugs Fixed

• Related to issue #

No ticket, just documentation fix

API Changes

None

Behavioral Changes

None

PR Checklist

• [x] Has tests (if omitted, state reason in description)
• [x] Has samples (if omitted, state reason in description)
• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [x] Updated documentation (see walkthrough)

[jfversluis]

Also you'll need to accept the CLA as mentioned in this comment.

[ivosnoza]

@microsoft-github-policy-service agree

[jfversluis]

Thank you so much!