Fix documentation error.

Summary:
This little error is causing the table of contents to be messed up.
See: https://facebook.github.io/watchman/docs/install.html
It has two entries on left side for 'watchman-wait' and is missing
an entry.

Reviewed By: wez

Differential Revision: D5835138

fbshipit-source-id: ea82e16fb12c55a03dc7e4a49c045e33587f6956

website/_docs/cppclient.markdown

@@ -1,5 +1,5 @@
 ---
-id: cppclient
+pageid: cppclient
 title: C++ Client
 layout: docs
 section: Invocation