Correct nbtsplitloc.c comment.

The logic just added by commit e3899ffd falls back on a 50:50 page split
in the event of a new item that's just to the right of our provisional
"many duplicates" split point.  Fix a comment that incorrectly claimed
that the new item had to be just to the left of our provisional split
point.

Backpatch: 12-, just like commit e3899ffd.

src/backend/access/nbtree/nbtsplitloc.c

@@ -815,7 +815,7 @@ _bt_bestsplitloc(FindSplitData *state, int perfectpenalty,
 	{
 		/*
 		 * Avoid the problem by peforming a 50:50 split when the new item is
-		 * just to the left of the would-be "many duplicates" split point.
+		 * just to the right of the would-be "many duplicates" split point.
 		 */
 		final = &state->splits[0];
 	}