Change url message to avoid confusion

Author: Robin Maenhaut

popup.html

@@ -44,7 +44,7 @@
       <button id="stop" type="button">Stop checking availability</button>
     </div>
     <div>
-      <p>Currently checking the following URLs:</p>
+      <p>Currently checking the following URLs (if opened in a tab):</p>
       <ul id="urls"></ul>
     </div>
   </body>