Fix comment of SimpleLruInit() in slru.c

sync_handler was not mentioned in the comment block of the function.

Oversight in dee663f.

Author: Aleksander Alekseev
Discussion: https://postgr.es/m/CAJ7c6TPUd9BwNY47TtMxaijLHSbyHNdhu=kvbGnvO_bi+oC6_Q@mail.gmail.com
Backpatch-through: 14

src/backend/access/transam/slru.c

@@ -181,6 +181,7 @@ SimpleLruShmemSize(int nslots, int nlsns)
  * ctllock: LWLock to use to control access to the shared control structure.
  * subdir: PGDATA-relative subdirectory that will contain the files.
  * tranche_id: LWLock tranche ID to use for the SLRU's per-buffer LWLocks.
+ * sync_handler: which set of functions to use to handle sync requests
  */
 void
 SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,