Added missing word and link

Based on the keypoints found we have to +extract+ features (linked tutorial which explains how features work in pcl)
Remove reference to openni-dev in pkg-config files.

OpenNI packages are not currently shipped with any pkg-config
files. Furthermore, Debian packages libopenni-dev and openni-dev are
currently not coinstallable and provide pkg-config files with
different namings. To workaround these issues, this patch disables
OpenNI check in pkg-config files.

This partially solves issue #221.
Merge pull request #295 from stanislas-brossette/master

Remove reference to openni-dev in pkg-config files.
Merge pull request #297 from georgebrindeiro/patch-1

Added missing word and link