Change `relative_roots` to `relative_root` in code samples.
Change `relative_roots` to `relative_root` in code samples.

Summary: This seems like a typo.

Reviewed By: wez

Differential Revision: D4411711

fbshipit-source-id: 51582f3758c4f4624ea1fdf32440b19f6cf1e57c

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