doc: update signal.Notify example to use buffered channel

This if follow up of CL 274332.

Updates #9399.

Change-Id: Ic6dd534dc18227a799cbb9577979f2285596b825
Reviewed-on: https://go-review.googlesource.com/c/go/+/274393
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: cuonglm

doc/go1.html

@@ -1647,14 +1647,14 @@ <h3 id="os_signal">The os/signal package</h3>
 is
 </p>
 <pre>
-c := make(chan os.Signal)
+c := make(chan os.Signal, 1)
 signal.Notify(c) // ask for all signals
 </pre>
 <p>
 but most code should list the specific signals it wants to handle instead:
 </p>
 <pre>
-c := make(chan os.Signal)
+c := make(chan os.Signal, 1)
 signal.Notify(c, syscall.SIGHUP, syscall.SIGQUIT)
 </pre>