Fix markdown headings

Fix markdown headings

GitHub recently changed how atx headings (beginning with `#`) are
rendered. A space is now required between `#` and the following text
for the heading to be recognized.

Update headings in the docs to match the expected format.

See https://github.github.com/gfm/#atx-headings

Commit migrated from https://github.com/dotnet/coreclr/commit/765f28934dc3c47e0f3658a818bcc04d3c71c9ef