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

Basically reverts rust-lang#25508. The `is_dir()` function has been stable since 1.5.0.

Author: kennytm

src/libstd/fs.rs

@@ -1338,10 +1338,10 @@ pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 ///
 /// // one possible implementation of walking a directory only visiting files
 /// fn visit_dirs(dir: &Path, cb: &Fn(&DirEntry)) -> io::Result<()> {
-///     if try!(fs::metadata(dir)).is_dir() {
+///     if dir.is_dir() {
 ///         for entry in try!(fs::read_dir(dir)) {
 ///             let entry = try!(entry);
-///             if try!(fs::metadata(entry.path())).is_dir() {
+///             if try!(entry.file_type()).is_dir() {
 ///                 try!(visit_dirs(&entry.path(), cb));
 ///             } else {
 ///                 cb(&entry);