Update API.md (#359)

Using-the-API/API.md

@@ -405,7 +405,13 @@ Returns an empty object.
 
 #### Dismissing a single notification:
 
-    POST /api/v1/notifications/dismiss/:id
+    POST /api/v1/notifications/dismiss
+    
+Form data:
+
+| Field | Description                   | Optional   |
+| ----- | ----------------------------  | ---------- |
+| `id`  | Notification ID | no         |    
 
 Deletes a single notification from the Mastodon server for the authenticated user.
 Returns an empty object.