Rollup merge of #37255 - mbrubeck:doc-edit, r=steveklabnik

Fix some mistakes in HRTB docs

The example code for higher-ranked trait bounds on closures had an unnecessary `mut` which was confusing, and the text referred to an mutable reference which does not exist in the code (and isn't needed).  Removed the `mut`s and fixed the text to better describe the actual error for the failing example.

Thanks to csd_ on IRC for pointing out these problems!

r? @steveklabnik

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