Fix typo.

Thomas Munro

src/backend/access/heap/heapam.c

@@ -3480,7 +3480,7 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
 	Assert(ItemPointerIsValid(otid));
 
 	/*
-	 * Forbid this during a parallel operation, lets it allocate a combocid.
+	 * Forbid this during a parallel operation, lest it allocate a combocid.
 	 * Other workers might need that combocid for visibility checks, and we
 	 * have no provision for broadcasting it to them.
 	 */