message: docs(migration): use function for this context
diff: @@ -100,7 +100,7 @@ The following example shows the kind of changes you will need to make in user-de
  Here is an example of a user-defined prototype operator:
  ```ts
  -Observable.prototype.userDefined = () => {
  +Observable.prototype.userDefined = function () {
  return new Observable((subscriber) => {
  this.subscribe({
  next(value) { subscriber.next(value); },
  @@ -108,7 +108,7 @@ Observable.prototype.userDefined = () => {
  complete() { subscriber.complete(); },
  });
  });
  -});
  +};
  source$.userDefined().subscribe();
  ```
  