term: use stdin file descriptor in comment

Using a `fd` of `0` on Windows will crash with `panic: The handle is invalid.`
By getting the file descriptor from `os.Stdin`, this can be avoided.

Change-Id: Ie7915b1e0e1018fcb92b6ba006fe85a9aee439ad
GitHub-Last-Rev: 4e56574120
GitHub-Pull-Request: golang/term#3
Reviewed-on: https://go-review.googlesource.com/c/term/+/280232
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

term.go

@@ -7,11 +7,11 @@
 //
 // Putting a terminal into raw mode is the most common requirement:
 //
-// 	oldState, err := term.MakeRaw(0)
+// 	oldState, err := term.MakeRaw(int(os.Stdin.Fd()))
 // 	if err != nil {
 // 	        panic(err)
 // 	}
-// 	defer term.Restore(0, oldState)
+// 	defer term.Restore(int(os.Stdin.Fd()), oldState)
 package term
 
 // State contains the state of a terminal.