Fixed example payload in protected branches API docs

What does this MR do?

id field in the example payload is not necessary and is in fact ignored (the id assigned to the protected branch is picked by GitLab). So this example is confusing as the reader does not know where this value came from. I think even the writer of this example got confused and assumed that the value is the project ID value, which is not true.

In fact, the whole existence of this id field is confusing and not explained in documentation at all. It does not seem to be necessary for any API call so it is unclear why it is exposed, even if internally exists in the database. So another issue should be to or hide the id field or document the id field.

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