Bug: Broken link in 'Mary Had a Little Synth' untutorial

For the [‘Mary Had a Little Synth’](https://coding.space/launchpad/703c1a8d-0c55-4bcb-b3ef-832eec05f138) project untutorial: The link for Scientific Pitch Notation in Step 6 is dead. Replace it with [this one](https://www.harpkit.com/resources/scientific-pitch-notation-explained) which is a brief primer with a helpful visual of the notes on a keyboard.