Update GridFieldExportButton.php

Quotes need to be escaped as "", not \":

http://tools.ietf.org/html/rfc4180#page-3 (point 7)

The whole function can probably be simplified a lot by using [fputcsv](http://php.net/manual/en/function.fputcsv.php) (like [this](http://www.zyxware.com/articles/3527/how-to-create-csv-files-using-php)), but I didn't want to risk breaking the custom functionality.