Fix typo on extensions page

[scrhartley]

Description

Typo:
"Serve Sent Events" should be "Server Sent Events" on the extensions page

Testing

Previewed in markdown viewer to verify that the table still renders correctly.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[Telroshan]

Thanks!