Docs: Fix some minor errors in tap dance example (#3530)

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