fixup: sphinx warning

Fixes the sphinx warning:
tools/clang/docs/DiagnosticsReference.rst:7889: WARNING: Title underline
too short.

That I just introduced in r350877.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350878 91177308-0d34-0410-b5e6-96231b3b80d8

docs/DiagnosticsReference.rst

@@ -7886,7 +7886,7 @@ This diagnostic is enabled by default.
 +-----------------------------------------------------------------------------------------------+-----------------------+
 
 -Woverride-init
---------------
+---------------
 Synonym for `-Winitializer-overrides`_.