Fixed install instructions
Fixed install instructions

Added php-xml, without it this issue can rise:
Class 'DOMDocument' not found in [/var/www/MISP/app/Lib/cakephp/lib/Cake/Utility/Xml.php
crruthe/MISP,SteveClement/MISP,devnull-/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,TheDr1ver/MISP,devnull-/MISP,MISP/MISP,TheDr1ver/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,Rafiot/MISP,crruthe/MISP,devnull-/MISP,MISP/MISP,cristianbell/MISP,devnull-/MISP,cristianbell/MISP,FloatingGhost/MISP,crruthe/MISP,FloatingGhost/MISP,FloatingGhost/MISP,SteveClement/MISP,MISP/MISP,FloatingGhost/MISP,MISP/MISP,Rafiot/MISP,TheDr1ver/MISP,FIRSTdotorg/MISP,cristianbell/MISP,Rafiot/MISP,SteveClement/MISP,TheDr1ver/MISP,Rafiot/MISP,FIRSTdotorg/MISP,cristianbell/MISP,crruthe/MISP