update ipython notebook sage extension instructions (suggested by Erik Massop)
update ipython notebook sage extension instructions (suggested by Erik Massop)

tscholl2/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc