doc: update import path for PEX

"I don't see any code still importing twitter.common.python pex (except for examples in rst files,which should probably be fixed)"

Bugs closed: 428

Reviewed at https://rbcommons.com/s/twitter/r/1298/
Sync of https://github.com/pantsbuild/pants/commit/44de6c1f4bc3f4b9da92405d1860db51a57ea612