Add missing comma in 2.11.0-M5 download page.

The missing comma made jekyll to fail.
Fix typos and turn github names and issues into links for 2.11.0-M5.

Fixed some typos and turned github names into links. Also, PR numbers
link to GitHub.

This also fixes problem with Jekyll which would fail to format release
notes page because of `- @adriaanm` construct. It seems like `@` has
some special meaning in Jekyll. Once github name has been turned into
a link everything started to work again.