page cache: Remove stray radix comment

Signed-off-by: Matthew Wilcox <willy@infradead.org>

mm/filemap.c

@@ -2682,7 +2682,7 @@ repeat:
 			put_page(page);
 			if (err == -EEXIST)
 				goto repeat;
-			/* Presumably ENOMEM for radix tree node */
+			/* Presumably ENOMEM for xarray node */
 			return ERR_PTR(err);
 		}