watchman: fix unknown attribute in doc
watchman: fix unknown attribute in doc

Summary:
Fix error while running tests:
```
error: unknown attribute `norun`. Did you mean `no_run`?
  --> /builddir/build/BUILD/watchman_client-0.7.1/src/lib.rs:1:1
```

Reviewed By: michel-slm

Differential Revision: D29804898

fbshipit-source-id: 48e2faa5a0d6fd13d9ff019e682a98ad682f8cdc

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