Resolve no-undef lint in build.js

book/build.js
   42:5   error  'path' is not defined  no-undef
   43:30  error  'path' is not defined  no-undef
   45:17  error  'path' is not defined  no-undef
   49:10  error  'path' is not defined  no-undef
   53:35  error  'path' is not defined  no-undef
  118:54  error  'path' is not defined  no-undef
  123:22  error  'path' is not defined  no-undef

✖ 7 problems (7 errors, 0 warnings)

Author: dtolnay

book/build.js

@@ -39,7 +39,7 @@ const dirs = ['build'];
 while (dirs.length) {
   const dir = dirs.pop();
   fs.readdirSync(dir).forEach((entry) => {
-    path = dir + '/' + entry;
+    const path = dir + '/' + entry;
     const stat = fs.statSync(path);
     if (stat.isDirectory()) {
       dirs.push(path);