Replace 'mutex' with 'lock' in RwLock documentation

[Kriskras99]

When copying the documentation for clear_poison from Mutex, not every occurence of 'mutex' was replaced with 'lock'.

[rustbot]

r? @thomcc

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[Kriskras99]

@thomcc can you review this? It's a small PR that only fixes the wording in the documentation.
(The documentation for the function was copied from the Mutex variant as can be seen in the original commit)

[thomcc]

@bors r+ rollup

[bors]

📌 Commit cdf25c9 has been approved by thomcc

It is now in the queue for this repository.