Fix missing api_key message

[kopchickm]

If filestack_rails.api_key is missing from the configuration, the error message says:

Set config.filepicker_rails.api_key

It should say:

Set config.filestack_rails.api_key

[coveralls]

Coverage remained the same at 96.414% when pulling dce164a on kopchickm:kopchickm-fix-api-msg into a7b6231 on filestack:master.

[gabifija]

@kopchickm nice catch, we will update it.