Fix typo and $PID example in README
Update README.md
Merge pull request #7 from derekchiang/patch-1

Update README.md
Merge branch 'patch-1' of github.com:JackDanger/huptime
Fix example.
Fix header guards (fixes #2).

TIL: Leading underscores followed by a capital letter is
actually a reserved identifier. This replaces the header
guards with a non-reserved identifier.