[Doc] Fix useRegisterMutationMiddleware documentation

[djhi]

Problem

Code examples in React-admin - useRegisterMutationMiddleware are erroneous because they do not return the next mutation result, although this is necessary for most main mutations handled by react admin (either create or update).

Solution

Fix the documentation.

Additional Checks

• The PR targets master for a bugfix, or next for a feature
  - [ ] The PR includes unit tests (if not possible, describe why)
  - [ ] The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

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