Fixing documentation

[tmaly1980]

I could be wrong, but I think the right way to install this plugin is via "ignite add json-server", not "yarn add json-server". The latter installs the 3rd party server package but doesn't call the ignite integration script. I think the documentation needs updating to reflect that.

[derekgreenberg]

Fixed. Thanks @tmaly1980 for finding and reporting this issue in the documentation.