updates plugin.views Reference.md entry to a clear and working example

[pletcher]

The current example for setting plugin-specific views engines breaks because require('jade') returns an object (and so extends and invalidates the options that get passed to pack.views). It also does not clearly show that "jade" refers to the file extension and not the module itself:

exports.register = function (plugin, options, next) {

    plugin.views({
        engines: {
            jade: require('jade') 
        },
        path: './templates'
    });

    next();
};

I'm proposing a simple change to the following, which tries to clarify that "html" refers to the file extension of the view template and also shows a working example that follows the guidelines specified in the docs:

exports.register = function (plugin, options, next) {

    plugin.views({
        engines: {
            html: { 
              module: require('handlebars') 
            }
        },
        path: './templates'
    });

    next();
};