Clarify the difference between get_mut and into_mut for OccupiedEntry

Clarify the difference between get_mut and into_mut for OccupiedEntry

The examples for both hash_map::OccupiedEntry::get_mut and
hash_map::OccupiedEntry::into_mut were almost identical. This led
to some confusion over the difference, namely why you would ever
use get_mut when into_mut gives alonger lifetime. Reddit thread:
https://www.reddit.com/r/rust/comments/8a5swr/why_does_hashmaps

This commit adds two lines and a comment to the example, to show
that the entry object can be re-used after calling get_mut.