fix apparent typo in fbcode_builder.py
fix apparent typo in fbcode_builder.py

Summary:
I noticed this while testing some changes; the comment
doesn't seem to match up to the `raise` statement, and attempting
to throw a list fails in python.

Reviewed By: strager

Differential Revision: D9566018

fbshipit-source-id: 563e43dfd30867993cc2a0f816db3b835a52e6df

wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,wez/watchman,wez/watchman,facebook/watchman,wez/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,wez/watchman,wez/watchman,nodakai/watchman