Docs: Fix some minor errors in tap dance example (#3530)
* Fix some minor errors in tap dance example
Fix for #3529
Fix minor errors in the code examples for __Example 4: 'Quad Function Tap-Dance'__ and relevant documentation.
Clarified the need to include the header file in `keymap.c`.
* Use #pragma once in header guard
Fix for #3529
Implement change requested in #3530