Update view engine jade example in the guide

[benrlodge]

On the jade example of changing the view engine in the guide under the settings heading, it would be nice to add that you also need to change:
this.engine('ejs', require('ejs').__express);
to
this.engine('jade', require('jade').__express);

as it's a little misleading to say

That is easily changed by setting the view engine option in config/environments/all.js:`

by just doing -
this.set('view engine', 'jade');

[crispen-smith]

+1

However, this once again raises the question of maintenance.