tools: replace string concatenation with template literals

Replace a string concatenation in the processList function
in doc/json.js.

If missing, initialize the textRow property in the same line of the
concatenation.

PR-URL: #16806
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/doc/json.js

@@ -256,8 +256,7 @@ function processList(section) {
                         `${JSON.stringify(tok)}\n` +
                         JSON.stringify(list));
       }
-      current.textRaw = current.textRaw || '';
-      current.textRaw += `${tok.text} `;
+      current.textRaw = `${current.textRaw || ''}${tok.text} `;
     }
   });