colornames: update link to SVG spec

Fixes golang/go#68473

Change-Id: I0441ca8f17fc22e43ba4c2e3213a74b9f7e24d09
Reviewed-on: https://go-review.googlesource.com/c/image/+/599835
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <nigeltao@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

colornames/colornames.go

@@ -6,5 +6,5 @@
 
 // Package colornames provides named colors as defined in the SVG 1.1 spec.
 //
-// See http://www.w3.org/TR/SVG/types.html#ColorKeywords
+// See https://www.w3.org/TR/SVG11/types.html#ColorKeywords
 package colornames