path/filepath: remove named result parameter for VolumeName

Fix style by removing unnecessary named result parameter.

Fix doc comment while here.

Change-Id: If8394e696ab37e00a95484d5137955aa06c59520
Reviewed-on: https://go-review.googlesource.com/1781Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/path/filepath/path.go

@@ -456,9 +456,9 @@ func Dir(path string) string {
 }
 
 // VolumeName returns leading volume name.
-// Given "C:\foo\bar" it returns "C:" under windows.
+// Given "C:\foo\bar" it returns "C:" on Windows.
 // Given "\\host\share\foo" it returns "\\host\share".
 // On other platforms it returns "".
-func VolumeName(path string) (v string) {
+func VolumeName(path string) string {
 	return path[:volumeNameLen(path)]
 }