Fix function name in comment.

Author: Masahiro Ikeda
Discussion: https://postgr.es/m/0043eee90b38351ea199d7e3294c10c4@oss.nttdata.com

src/backend/utils/cache/relfilenodemap.c

@@ -82,7 +82,7 @@ RelfilenodeMapInvalidateCallback(Datum arg, Oid relid)
 }
 
 /*
- * RelfilenodeMapInvalidateCallback
+ * InitializeRelfilenodeMap
  *		Initialize cache, either on first use or after a reset.
  */
 static void