message: Fix an example in SequenceChecker documentation
  SequenceChecker needs to be prefixed with & in RTC_DCHECK_RUN_ON;
  all examples except the first one were showing this.
  Cr-Commit-Position: refs/heads/master@{#33771}
diff: @@ -26,7 +26,7 @@ namespace webrtc {
  // class MyClass {
  // public:
  // void Foo() {
  -// RTC_DCHECK_RUN_ON(sequence_checker_);
  +// RTC_DCHECK_RUN_ON(&sequence_checker_);
  // ... (do stuff) ...
  // }
  //
  