net/http/fcgi: update URL to spec documentation

Fixes #23748

Change-Id: Iacdbd57c53492175cf1d3045640db61151e10731
Reviewed-on: https://go-review.googlesource.com/92955Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/http/fcgi/fcgi.go

@@ -4,9 +4,8 @@
 
 // Package fcgi implements the FastCGI protocol.
 //
-// The protocol is not an official standard and the original
-// documentation is no longer online. See the Internet Archive's
-// mirror at: https://web.archive.org/web/20150420080736/http://www.fastcgi.com/drupal/node/6?q=node/22
+// See https://fast-cgi.github.io/ for an unofficial mirror of the
+// original documentation.
 //
 // Currently only the responder role is supported.
 package fcgi