Fix hint wording in tutorial.md

[emshotton]

The tutorial hints that a change can be made that allows you to go back
in time then click in the board to create a new entry, but the change is
already present in the example code.

This fix removes the hint and re-words the explanation of the change the
example code is making.

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure the test suite passes (npm test).
5. Make sure your code lints (npm run lint).
6. Format your code with prettier (npm run prettier).
7. Run the Flow typechecks (npm run flow).
8. If you added or removed any tests, run ./scripts/fiber/record-tests before submitting the pull request, and commit the resulting changes.
9. If you haven't already, complete the CLA.

[sophiebits]

Works for me, thank you!