Fix incorrectly-reverted change to mkdirSync call

An intermediate version of this code read:

    fs.mkdir(demoStaticTmpDir, {recursive: true}), done);

but apparently `fs.mkdir` doesn't honour the recursive option, so I
tried to revert the change but munged it instead.

This commit cleans up the mess I made.

scripts/gulpfiles/appengine_tasks.js

@@ -28,7 +28,7 @@ function prepareDeployDir(done) {
   if (fs.existsSync(demoTmpDir)) {
     rimraf.sync(demoTmpDir);
   }
-  fs.mkdir(demoStaticTmpDir, {recursive: true});
+  fs.mkdirSync(demoStaticTmpDir, {recursive: true});
   done();
 }