[DOC] Updates the signature for configuration properties as functions

[xcambar]

What Changed & Why

The doc at http://ember-cli-deploy.com/docs/v1.0.x/configuration/#usage states that:

Individual config properties can be pure values or functions that receive the context object and return a value.

// config/deploy.js
module.export = function(deployTarget){
 var ENV = {
   redis: {
     revisionKey: function(context) {
       return context.deployment.tag;
     }
   }
 }
};

While the code says differently: https://github.com/ember-cli-deploy/ember-cli-deploy-plugin/blob/master/index.js#L68-L70

This PR amends the documentation to reflect the current behaviour.

PR Checklist

• Add tests
• Add documentation
• Prefix documentation-only commits with [DOC]

[lukemelia]

In the code you're referring to: configuredValue.call(this.pluginConfig, this.context, _pluginHelper.call(this)), the first argument to call is what becomes this inside the function, not an argument to the function.

[xcambar]

Oh no... I missed the .call.

So this is what coding during a paternity leave looks like 😂

[xcambar]

I've amended to only edit the example.
Let's 🤞 that I didn't miss anything this time :)

[achambers]

I'm not sure the proposed change is actually accurate. I need to refamiliarise myself with the code and come back to this with more context though.