Update docs to reflect working monkey-patch method

Monkey-patching changed post https://github.com/jenkinsci/job-dsl-plugin/commit/3bf55f83e1087f2f2ca945d353e869884178cf66
and you need to `use(common) { job { ... } }` now.
Merge pull request #185 from nowells/extending-dsl-docs

Update docs to reflect working monkey-patch method