Fix chapter nesting of Gerrit v3.12 release notes

The Gerrit v3.12 release notes had the main header sections incorrectly
labelled as sub-headers within the security fixes chapter.

Rearrange the nesting of the fixes and align with the other release
notes.

Change-Id: Id7587e93ab87048d2d8150f7a396788221592177
1 file changed