config: fix typo

config.js

     /**
      External API url used to receive branding specific information.
      If there is no url set or there are missing fields, the defaults are applied.
-     None of the fieds are mandatory and the response must have the shape:
+     None of the fields are mandatory and the response must have the shape:
      {
          // The hex value for the colour used as background
          backgroundColor: '#fff',