Update site-design-json-schema.md

remove link from linkless heading example. 
This is to allow the adding of headings to the navigation that don't go anywhere.

Author: westleyMS

docs/declarative-customization/site-design-json-schema.md

@@ -641,7 +641,7 @@ Use the `addNavLink` verb to add a new navigation link to the site QuickLaunch o
 },
 {
   "verb": "addNavLink",
-  "url": "http://linkless.header/",
+  "url": "",
   "displayName": "Linkless Heading",
   "isWebRelative": false
 }