docs: Update description for format.formatter (#363)

Make it clear that the function is bound to the file object.

lib/register/format.js

@@ -18,7 +18,7 @@
  * @param {Object} format
  * @param {String} format.name - Name of the format to be referenced in your config.json
  * @param {Function} format.formatter - Function to perform the format. Takes 2 arguments, `dictionary` and `config`
- * Must return a string.
+ * Must return a string. Function is bound to its file object in the `platform.files` array.
  * @returns {module:style-dictionary}
  * @example
  * ```js