Specify WCAG-2.1 branch in respec github config (#4266)

This updates the respec config for WCAG 2.1 to reference this `WCAG-2.1`
branch, which impacts the commit history link included in the details at
the top of the document.

Author: kfranqueiro

guidelines/respec-config.js

@@ -41,7 +41,10 @@ var respecConfig = {
 
   // Name of the WG
   group: "ag",
-  github: "w3c/wcag",
+  github: {
+    repoURL: "https://github.com/w3c/wcag",
+    branch: "WCAG-2.1"
+  },
 
 	// editors, add as many as you like
 	// only "name" is required