[iAD] Fix message. fixes #12569

[mandel-macaque]

Add the missing '.'.

fixes #12569

[dalexsoto]

@mandel-macaque did you run the tests? just checking we are not missing any other ones.

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests failed on Build ❌

Tests failed on Build.

• Azure DevOps
• Html Report (VSDrops)

API diff

✅ API Diff from stable

View API diff

• iOS vsdrops gist
• macOS vsdrops gist
• macCat vsdrops gist
• macCatiOS gist
• tvOS vsdrops gist
• watchOS vsdrops gist

API & Generator diff

✅ API Diff (from PR only) (no change)
✅ Generator Diff (only version changes)

GitHub pages

Results can be found in the following github pages (it might take some time to publish):

• Test results
• API diff
• API & Generator diff

Test results

2 tests failed, 106 tests passed.

Failed tests

• dont link/Mac Catalyst/Debug [dotnet]: Failed (Test run crashed (exit code: 134).
  Tests run: 11 Passed: 6 Inconclusive: 0 Failed: 0 Ignored: 5)
• framework-test/Mac Catalyst/Debug [dotnet]: TimedOut (Execution timed out after 1200 seconds.
  No test log file was produced)

Pipeline on Agent XAMBOT-1104.BigSur'
Merge 3cfa19e into 8eab757

[mandel-macaque]

Tests did not fail, the mac app crashed but is not related to the PR.