fix permalink to docs/install.html
fix permalink to docs/install.html

Summary:
With the new updates, the "docs" nav link at the top of  http://facebook.github.io/watchman/docs/install.html referred to `http://facebook.github.io/watchman/docs/docs/install.html`.
Closes https://github.com/facebook/watchman/pull/418

Differential Revision: D4394233

Pulled By: sid0

fbshipit-source-id: 88b166ff4458070bb6660e24d347590a3ee491f9

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