Fix predicate signatures in retain_mut docs

[collinoc]

This is my first PR here so let me know if I'm doing anything wrong.

The docs for retain_mut in LinkedList and VecDeque say the predicate takes &e, but it should be &mut e to match the actual signature. Vec has it documented correctly already.

[rustbot]

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[jhpratt]

Nope, this looks change looks good. You're absolutely correct that this is a mistake, mostly likely from copy-pasting the docs when it was first implemented.

@bors r+ rollup

[bors]

:pushpin: Commit 3ed5d5590e12d972acf4ca6a503d0b00f233ad0e has been approved by jhpratt

It is now in the queue for this repository.