Fix typo with wait event for SLRU buffer of commit timestamps

Fix typo with wait event for SLRU buffer of commit timestamps

This wait event was documented as "CommitTsBuffer" since its
introduction, but the code named it "CommitTSBuffer".  This commit fixes
the code to follow the term documented, which is also more consistent
with the naming of the other wait events used for commit timestamps.

Introduced by 5da1493.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/e8c38840-596a-83d6-bd8d-cebc51111572@gmail.com
Backpatch-through: 13