add localhost to google recaptcha code example

Example: recaptcha api key for localhost

"With the following test keys, you will always get No CAPTCHA and all verification requests will pass.

Site key: 6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI

Secret key: 6LeIxAcTAAAAAGG-vFI1TnRWxMZNFuojJ4WifJWe

The reCAPTCHA widget will show a warning message to claim that it's only for testing purpose. Please do not use these keys for your production traffic."

Extracted from here: https://developers.google.com/recaptcha/docs/faq#id-like-to-run-automated-tests-with-recaptcha.-what-should-i-do