RelNotes: name the release properly

In the title, we should state for which version this release notes
document is about.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.20.0.txt

-Git Release Notes
-=================
+Git 2.20 Release Notes
+======================
 
 Backward Compatibility Notes
 ----------------------------