Fixes a typo in the HashMap doc

[TisButMe]

I bumped into this yesterday while writing some hashmap code.

This is minor, feel free to ignore if you end up removing the method anyway.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

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[TisButMe]

The code seems to actually use cloned(), I'll look into into a bit more before reopening.

[sfackler]

cloned() turns an Option<&T> and returns an Option<T> by cloning the T inside. clone() just takes an Option<&T> and returns a new Option<&T>.

[TisButMe]

Thanks for the info :) I wondered where was cloned coming from.