Summary:
The commands as they were previously would attempt to install Folly as 32-bit, which would fail.
Also removes an extra `-` in the link's url.

Closes: https://github.com/facebook/folly/issues/713

Reviewed By: yfeldblum

Differential Revision: D6327981

fbshipit-source-id: ced8c86948b8be5c1dd88e14d6c3a77fc783aac9