doc: add comma for better understanding (#145)

API.md

@@ -56,7 +56,7 @@ app
 <a name="module_koa-router--Router+get|put|post|patch|delete|del"></a>
 
 #### router.get|put|post|patch|delete|del ⇒ <code>Router</code>
-Create `router.verb()` methods, where *verb* is one of the HTTP verbs such
+Create `router.verb()` methods, where *verb* is one of the HTTP verbs, such
 as `router.get()` or `router.post()`.
 
 Match URL patterns to callback functions or controller actions using `router.verb()`,