Fix WP_CLI_ROOT

In 3ca6c14ab22042ce3a1be78a5ac057ec0c735358 I fixed the output from `wp --info`, but maybe we should fix WP_CLI_ROOT instead.

We use it to access files from `man-src` and `man`, not just stuff in the `php/` dir.

While we're at it, we could also remove the trailing slash.

Related: #439