os: fix typo in comment (Readir -> Readdir)

Change-Id: I8434925661dc11396380af65c192c9f0dc191287
Reviewed-on: https://go-review.googlesource.com/27973Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/dir_unix.go

@@ -35,7 +35,7 @@ func (f *File) readdir(n int) (fi []FileInfo, err error) {
 		fi = append(fi, fip)
 	}
 	if len(fi) == 0 && err == nil && n > 0 {
-		// Per File.Readir, the slice must be non-empty or err
+		// Per File.Readdir, the slice must be non-empty or err
 		// must be non-nil if n > 0.
 		err = io.EOF
 	}