Correct AUTH_HEADER_NAME in docs (#725)

The correct is X-Access-Token e to AUTH_HEADER_NAME example and not  X_Access_Token

Author: chicosilva

docs/settings.rst

@@ -191,7 +191,7 @@ collection will be used to build the "WWW-Authenticate" header in the response.
 The authorization header name to be used for authentication.
 The default is ``HTTP_AUTHORIZATION`` which will accept the
 ``Authorization`` header in the request. For example if you'd
-like to use ``X_Access_Token`` in the header of your requests
+like to use ``X-Access-Token`` in the header of your requests
 please specify the ``AUTH_HEADER_NAME`` to be
 ``HTTP_X_ACCESS_TOKEN`` in your settings.