Minor fix base/bind.h documentation.

|base::OnceCallback::Run| can only be called on rvalue reference.

Bug: None
Change-Id: I209de9b09c4b37ed97f592f6a077b9c4d8152821
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1646013
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666442}

base/bind.h

@@ -42,7 +42,7 @@
 //   class C : public base::RefCounted<C> { void F(); };
 //   auto instance = base::MakeRefCounted<C>();
 //   auto cb = base::BindOnce(&C::F, instance);
-//   cb.Run();  // Identical to instance->F()
+//   std::move(cb).Run();  // Identical to instance->F()
 //
 // base::Bind is currently a type alias for base::BindRepeating(). In the
 // future, we expect to flip this to default to base::BindOnce().