Fix inconsistent LWLock tranche name "CommitTsSLRU"

Fix inconsistent LWLock tranche name "CommitTsSLRU"

This term was using an inconsistent casing between the code and the
documentation, using "CommitTsSLRU" in wait_event_names.txt and
"CommitTSSLRU" in the code.

Let's update the term in the code to reflect what's in the
documentation, "CommitTs" being more commonly used, so as
pg_stat_activity shows the same term as the documentation.

Oversight in 53c2a97a9266.

Author: Alexander Lakhin
Discussion: http://postgr.es/m/f7e514cf-2446-21f1-a5d2-8c089a6e2168@gmail.com
Backpatch-through: 17