documentation: fixes in install instructions

This is a first pull request to get acquainted with the Invenio development cycle.  It fixes small typos that I noticed in the install instructions.

Besides this, it is worth noting that the instructions regarding nodejs and npm are no longer required in Ubuntu 14.04. In my case, everything went smoothly by simply installing the `nodejs`, `nodejs-legacy` and `npm` packages directly from the official Ubuntu repo. What version of Ubuntu do you officially support? 

Signed-off-by: Gilles Louppe <g.louppe@gmail.com>