Improve header comment for LockHasWaitersRelation.

Dimitry Ivanov spotted a typo, and I added a bit of wordsmithing.

src/backend/storage/lmgr/lmgr.c

@@ -268,8 +268,8 @@ UnlockRelation(Relation relation, LOCKMODE lockmode)
 /*
  *		LockHasWaitersRelation
  *
- * This is a functiion to check if someone else is waiting on a
- * lock, we are currently holding.
+ * This is a function to check whether someone else is waiting for a
+ * lock which we are currently holding.
  */
 bool
 LockHasWaitersRelation(Relation relation, LOCKMODE lockmode)