Update hash-map.dd

Typo that generated a bug for at least one user.

<Destructionator> see if it works with toHash instead of opHash
<Destructionator> i think it is supposed to be toHash
<lws> http://dlang.org/hash-map.html <-- stupid doc
<Destructionator> i could be wrong too
<lws> Yep that worked
<lws> thanks destruct
<lws> I copy and pasted from the doc and didn't notice that