Mention Bug Fix in release notes

The Git Bash prompt [now works even after calling `set
-u`](git-for-windows/git#2800).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

ReleaseNotes.md

@@ -65,6 +65,7 @@ Also, the option to override the branch name used by `git init` for the initial
 * Non-ASCII output of paged Git commands [is now rendered correctly in Windows Terminal](https://github.com/git-for-windows/git/pull/2834).
 * It is [now possible](https://github.com/git-for-windows/build-extra/pull/303) to use `wordpad.exe` as Git's editor of choice.
 * When using Git via the "Run As..." function, [it now uses the correct home directory](https://github.com/git-for-windows/git/pull/2725).
+* The Git Bash prompt [now works even after calling `set -u`](https://github.com/git-for-windows/git/pull/2800).
 
 ## Changes since Git for Windows v2.27.0 (June 1st 2020)