Fix missing <T> in sub header on async-in-depth.md

[Paxxi]

Summary

Describe your changes here.
Change to html escaped syntax to avoid having it removed during build.

Fixes #Issue_Number (if available)

[rpetrusha]

Thanks, @Paxxi, for contributing to the dotnet/docs repo and fixing the header. We'll merge your PR now. Your change should appear live on docs.microsoft.com in the next day or two.