doc: simplify urlObject.hash description

It's an octothorp, but no one knows what that is either so let's just
show the character in the sentence and be done with it.

PR-URL: https://github.com/nodejs/node/pull/22326


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>