std: small doc fixes for BufReader and BufWriter

[birkenfeld]

• fix probable copy-paste error in BufWriter.get_mut()
• more consistent punctuation

[rust-highfive]

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[steveklabnik]

Convention is to use full sentences, actually, so I'd prefer to leave this (and others)

[steveklabnik]

Thanks @birkenfeld ! I'll take the typo fixes, but the periods should stay.

[birkenfeld]

I don't understand - I'm adding the periods, which are also present in the first sentences of other docstrings in this file.

[huonw]

r? @steveklabnik

[steveklabnik]

@birkenfeld sorry! I guess I read the diff backwards. Periods are what we want, for sure.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit df73abe has been approved by steveklabnik