I believe the actual form HTML served by reCaptcha has being generated from your reCaptcha account at reCaptcha.
See:
http://rtfm.modx.com/display/ADDON/FormIt.Hooks.recaptcha for documentation on reCaptcha in MODX
and
http://rtfm.modx.com/display/ADDON/FormIt.Examples.Simple+Contact+Page for documentation on FormIt.