Mention `as_mut` alongside `as_ref` in borrowck error message

[compiler-errors]

Kinda fixes #99426 but I guess that really might be better staying open to see if we could make it suggest as_mut in a structured way. Not sure how to change borrowck to know that info tho.

[rust-highfive]

r? @fee1-dead

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[fee1-dead]

@bors r+

[bors]

:pushpin: Commit e37565d2db8d7cf92a6d1df535343d4b2881fc35 has been approved by fee1-dead

It is now in the queue for this repository.

[compiler-errors]

@bors rollup