Update variable declaration

Preferable to use 'let' instead of 'var' for variable declaration

Extensions/chrome/return-youtube-dislike.content-script.js

@@ -1,5 +1,5 @@
 chrome.runtime.sendMessage({}, (securityToken) => {
-  var script = document.createElement("script");
+  let script = document.createElement("script");
   script.setAttribute("security-token", securityToken);
   script.setAttribute("extension-id", chrome.runtime.id);
   script.src = chrome.runtime.getURL("return-youtube-dislike.script.js");