Invalid printf string

Hi!

Your plugin includes a translatable string within sprintf which does not work.

https://translate.ww.wp.xz.cn/projects/wp-plugins/wp-optimize/dev/es-co/default/?filters%5Bstatus%5D=either&filters%5Boriginal_id%5D=8981638&filters%5Btranslation_id%5D=83944130

return sprintf(__('A cache purge request has been sent to %s. Please note that it may not work 100% of the time, due to cache rate limiting by your host!', 'wp-optimize'), '<strong>WP Engine</strong>');

The percent symbol must be escaped as %% when included within a string that is passed to printf, currently this just returns false instead and as a result this string is never seen by users.