fix typo

Author: Minishlink

README.md

@@ -124,7 +124,7 @@ Urgency can be either "very-low", "low", "normal", or "high". If the browser ven
 #### topic
 Similar to the old `collapse_key` on legacy GCM servers, this string will make the vendor show to the user only the last notification of this topic (cf. [protocol](https://tools.ietf.org/html/draft-ietf-webpush-protocol-08#section-5.4)).
 
-#### Server errors
+### Server errors
 You can see what the browser vendor's server sends back in case it encountered an error (push subscription expiration, wrong parameters...).
 `sendNotification()` (with flush as true) and `flush()` returns true if there were no errors. If there are errors it returns an array like the following.
 The `expired` key can be useful to clean your database of expired endpoints.