Fix urbi-root in the installation guide.
Fix urbi-root in the installation guide.

	* doc/guide/installation.tex: Replace urbi_root by urbi-root to
	refer to the installation directory.

urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi