(get_)the_post_thumbnail uses $size='thumbnail'

The Snippets for the_post_thumbnail() and get_the_post_thumbnail() used a parameter $size = 'post-thumbnail', which isn't as size registered by default. Changing it to $size = 'thumbnail' would retrieve the attached file as thumbnail instead.
Merge pull request #3 from pixolin/patch-2

(get_)the_post_thumbnail uses $size='thumbnail'