tools: replace string concetation with templates

Replace string concatenation in tools/doc/html.js with template
literals.

PR-URL: #16801
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/doc/html.js

@@ -499,12 +499,12 @@ function buildToc(lexed, filename, cb) {
 
     depth = tok.depth;
     const realFilename = path.basename(realFilenames[0], '.md');
-    const id = getId(realFilename + '_' + tok.text.trim());
+    const id = getId(`${realFilename}_${tok.text.trim()}`);
     toc.push(new Array((depth - 1) * 2 + 1).join(' ') +
              `* <span class="stability_${tok.stability}">` +
              `<a href="#${id}">${tok.text}</a></span>`);
-    tok.text += '<span><a class="mark" href="#' + id + '" ' +
-                'id="' + id + '">#</a></span>';
+    tok.text += `<span><a class="mark" href="#${id}"` +
+                `id="${id}">#</a></span>`;
   });
 
   toc = marked.parse(toc.join('\n'));
@@ -518,7 +518,7 @@ function getId(text) {
   text = text.replace(/^_+|_+$/, '');
   text = text.replace(/^([^a-z])/, '_$1');
   if (idCounters.hasOwnProperty(text)) {
-    text += '_' + (++idCounters[text]);
+    text += `_${(++idCounters[text])}`;
   } else {
     idCounters[text] = 0;
   }