Use Path::is_dir() in fs::read_dir()'s example.
Use Path::is_dir() in fs::read_dir()'s example.

Basically reverts #25508. The `is_dir()` function has been stable since 1.5.0.
graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust