Fix copy code button interaction with kramdown line numbers (#1143)

Co-authored-by: Simone <26844016+simonebortolin@users.noreply.github.com>

assets/js/just-the-docs.js

@@ -504,7 +504,7 @@ jtd.onReady(function(){
 
     copyButton.addEventListener('click', function () {
       if(timeout === null) {
-        var code = codeBlock.querySelector('pre:not(.lineno)').innerText;
+        var code = (codeBlock.querySelector('pre:not(.lineno, .highlight)') || codeBlock.querySelector('code')).innerText;
         window.navigator.clipboard.writeText(code);
 
         copyButton.innerHTML = svgCopied;