version/go1.11beta1: change URL for release notes

Previously, the referenced URL for release notes was
https://golang.org/doc/go1.11beta1, where there was no document. This
change points it to https://tip.golang.org/doc/go1.11.

Updates golang/go#26240

Change-Id: Ie42cd9c16bc8ebe9e8f5cc820ff702f577212067
Reviewed-on: https://go-review.googlesource.com/122409
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: smasher164

version/go1.11beta1/main.go

@@ -12,7 +12,7 @@
 // And then use the go1.11beta1 command as if it were your normal go
 // command.
 //
-// See the release notes at https://golang.org/doc/go1.11beta1
+// See the release notes at https://tip.golang.org/doc/go1.11
 //
 // File bugs at https://golang.org/issues/new
 package main