Update example fs.md and use target_family instead of target_os

using target_family make code more compatible with different types of unix systems

src/std_misc/fs.md

@@ -7,9 +7,9 @@ use std::fs;
 use std::fs::{File, OpenOptions};
 use std::io;
 use std::io::prelude::*;
-#[cfg(target_os = "unix")]
+#[cfg(target_family = "unix")]
 use std::os::unix;
-#[cfg(target_os = "windows")]
+#[cfg(target_family = "windows")]
 use std::os::windows;
 use std::path::Path;
 
@@ -65,12 +65,12 @@ fn main() {
 
     println!("`ln -s ../b.txt a/c/b.txt`");
     // Create a symbolic link, returns `io::Result<()>`
-    #[cfg(target_os = "unix")] {
+    #[cfg(target_family = "unix")] {
         unix::fs::symlink("../b.txt", "a/c/b.txt").unwrap_or_else(|why| {
             println!("! {:?}", why.kind());
         });
     }
-    #[cfg(target_os = "windows")] {
+    #[cfg(target_family = "windows")] {
         windows::fs::symlink_file("../b.txt", "a/c/b.txt").unwrap_or_else(|why| {
             println!("! {:?}", why.to_string());
         });