Fix documentation in callback.h

Comments in callback.h were confusing because sample code was
incorrect. Updated sample code to make it consistent with the
documented behavior.

Change-Id: Id42a9465c49b65937d7b34bf8c0b3664085a628f
Reviewed-on: https://chromium-review.googlesource.com/1019870Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552205}

base/callback.h

@@ -34,7 +34,8 @@
 //   }, 1);
 //   // Run() only needs the remaining unbound argument |y|.
 //   printf("1 + 2 = %d\n", std::move(cb).Run(2));  // Prints 3
-//   printf("cb is null? %s\n", cb ? "true" : "false");  // Prints true
+//   printf("cb is null? %s\n",
+//          cb.is_null() ? "true" : "false");  // Prints true
 //   std::move(cb).Run(2);  // Crashes since |cb| has already run.
 //
 // Callbacks also support cancellation. A common use is binding the receiver