fix typo

Author: dunglas

mercure.rst

@@ -430,7 +430,7 @@ The supported options are:
 
 * ``subscribe``:  the list of topic selectors to include in the ``mercure.subscribe`` claim of the JWT
 * ``publish``:  the list of topic selectors to include in the ``mercure.publish`` claim of the JWT
-* ``additionalClaims``: exra claims to include in the JWT (expiration date, token ID...)
+* ``additionalClaims``: extra claims to include in the JWT (expiration date, token ID...)
 
 Using cookies is the most secure and preferred way when the client is a web
 browser. If the client is not a web browser, then using an authorization header