Correct the JSBin version

Fixes #5140

This version of the URL matched CONTRIBUTING.md

Author: Westbrook

.github/ISSUE_TEMPLATE.md

@@ -10,7 +10,7 @@ If you are asking a question rather than filing a bug, try one of these instead:
 
 #### Live Demo
 <!-- jsBin starting point (fork and edit) -->
-https://jsbin.com/luhaxab/1/edit
+https://jsbin.com/luhaxab/edit
 <!-- glitch.me starting point (remix and edit -- must be logged in to persist!) -->
 https://glitch.com/edit/#!/polymer-repro?path=my-element.html:2:0
 <!-- ...or provide your own repro URL -->