Spelling fix

Trivial spelling fix.

in stem.prereq check_requriements should be check_requirements

Diff to fix here https://github.com/eoinof/stem/commit/f9f0907201e19f24abee3a452a95782d03faefb9

I'm quite happy to add any more spelling fixes to this ticket before anything gets done..

Trac:
Username: eoinof