Set a better example in the docs

Adapted from an original patch from @michbsd: on top of normalizing the
host header we now have a regular expressions that only match the right
domains and potential sub-domains.

Closes #2140

doc/sphinx/users-guide/vcl-separate.rst

@@ -28,14 +28,19 @@ Next we write the top-level VCL program, which branches out
 to the other two, depending on the Host: header in the
 request::
 
-    /* We have to have a backend, even if we do not use it */
+    import std;
+
+    # We have to have a backend, even if we do not use it
     backend default { .host = "127.0.0.1"; }
 
     sub vcl_recv {
-	if (req.http.host ~ "varnish.org$") {
+	# Normalize host header
+	set req.http.host = std.tolower(req.http.host);
+
+	if (req.http.host ~ "\.?varnish\.org$") {
 	    return (vcl(l_vo));
 	}
-	if (req.http.host ~ "varnish-cache.org$") {
+	if (req.http.host ~ "\.?varnish-cache\.org$") {
 	    return (vcl(l_vc));
 	}
 	return (synth(302, "http://varnish-cache.org"));