[docs] Fix reference to obsolete fs::stat function

[mbrubeck]

This function was renamed to fs::metadata.

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[crumblingstatue]

fs::stat is also mentioned here:

rust/src/libstd/path.rs

Lines 1819 to 1824 in 1be3f9f

	/// Boolean value indicator whether the underlying file exists on the local
	/// filesystem. Returns false in exactly the cases where `fs::stat` fails.
	#[stable(feature = "path_ext", since = "1.5.0")]
	pub fn exists(&self) -> bool {
	fs::metadata(self).is_ok()
	}

[steveklabnik]

@mbrubeck mind snagging that other one while you're at it?

[alexcrichton]

Oh wow oops I just decided to clean out some branches in rust-lang/rust and apparently this was created as a branch from there!

Sorry about that, but could you reopen as a branch from your own repo?