getting started codelab part 6 document improvements.

[Berttwm]

Some documentation improvements for Part 6 (Creating a custom block), this particular part can be appended with additional details as explained below.

It is important to also add the <script src="scripts/sound_blocks.js"></script> before <script src="scripts/main.js"></script> as doing otherwise will not even show the "play" box as intended. I was stuck on this issue until I took a look at the ./complete-code/ folder