Proto/Scoru: improve remember's spec

Context

It is not expected to happen in practice, but in theory, when could ask to remember a binding ptr -> cell in a history h where ptr actually already appears in h.

This tiny MR explicitly specifies this case: h is returned unchanged.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR