fix commentary

store.el

@@ -34,9 +34,9 @@
 ;; - List operations: `store-push', `store-delete'
 ;; See their docstrings for details.
 
-;; The package supports multiple backend.  All backends should persist
-;; all changes immediately into external storage.  It comes with two
-;; built-in backend:
+;; The package supports multiple backends.  All backends should
+;; persist all changes immediately into external storage.  It comes
+;; with the following built-in backend:
 
 ;; - `log': Log-structured incremental S-expr store.  Only increments
 ;;   are stored, therefore the cost for each persistent operation