Fixed code indentation

Hello,

This is a fix for a really minor classical formatting problem in the project README.md file.

To see the problem, with an HTML browser, view the README.md file of the master branch as rendered by the GitHub markdown engine: the indentation is wrong for the ajaxFileUploader.

The problem comes from mixing tabs and spaces.

So the proposed fixed is using only spaces for indentation for the ajaxFileUploader example.

Cheers,