Fix documentation syntax error (#519)

Author: dtalkachou

docs/add_custom_data_claims.rst

@@ -29,8 +29,8 @@ going to the data supplied by the ``additional_claims`` argument.
   # claims to add to the JWT.
   @jwt.additional_claims_loader
   def add_claims_to_access_token(identity):
-       return = {
-           "aud": "some_audience",
-           "foo": "bar",
-           "upcase_name": identity.upper(),
-       }
+      return {
+          "aud": "some_audience",
+          "foo": "bar",
+          "upcase_name": identity.upper(),
+      }