Error from the example of mutation provided in the documentation

Following the [documentation](https://github.com/drupal-graphql/graphql/blob/8.x-4.x/doc/mutations/README.md), I wanted to use demo queries and mutations provided in
[this repository](https://github.com/joaogarin/mydrupalgql).

After fixing the issue related in https://github.com/joaogarin/mydrupalgql/issues/2, we still have an error while requesting a mutation:

`Missing input mapper for argument data.`

I think it would be nice having a working example that includes mutations (the examples provided in the graphql module contain queries only). What do you think?