Update customizing_token_claims.rst (#714)

In ""TOKEN_OBTAIN_SERIALIZER": "rest_framework_simplejwt.serializers.MyTokenObtainPairSerializer"," replaced "rest_framework_simplejwt" to "my_app" to make it clearer that it should be a custom path, since the Django app folder having the same name as the library was confusing and hard to fix if copy and pasting in a hurry.

docs/customizing_token_claims.rst

@@ -32,7 +32,7 @@ generated by the ``TokenObtainPairView``:
 
   SIMPLE_JWT = {
     # It will work instead of the default serializer(TokenObtainPairSerializer).
-    "TOKEN_OBTAIN_SERIALIZER": "rest_framework_simplejwt.serializers.MyTokenObtainPairSerializer",
+    "TOKEN_OBTAIN_SERIALIZER": "my_app.serializers.MyTokenObtainPairSerializer",
     # ...
   }