Remove from docs the "post_type" filter parameter for /posts endpoint

The post type must be passed as a ```type``` parameter with the GET request, not as a query variable for for the ```filter``` parameter.

Post types are pulled from ```type``` parameter [here](https://github.com/WP-API/WP-API/blob/develop/lib/class-wp-json-posts.php#L206-L214) and any ```post_type``` query variable in the ```filter``` parameter is discarded [here](https://github.com/WP-API/WP-API/blob/develop/lib/class-wp-json-posts.php#L241).

```type``` parameter is already documented a little farther down in the docs.