Fix typo

Author: ebarnard

src/libstd/fs.rs

@@ -1582,7 +1582,7 @@ pub fn rename<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<()>
 /// On Windows, this function currently corresponds to `CopyFileEx`. Alternate
 /// NTFS streams are copied but only the size of the main stream is returned by
 /// this function. On MacOS, this function corresponds to `copyfile` with
-/// `COPYFILE_ALL`
+/// `COPYFILE_ALL`.
 /// Note that, this [may change in the future][changes].
 ///
 /// [changes]: ../io/index.html#platform-specific-behavior