Fix typo in docs/ref_functions.md (#16690)

docs/ref_functions.md

@@ -1,6 +1,6 @@
 # List of Useful Core Functions To Make Your Keyboard Better
 
-There are a lot of hidden functions in QMK that are incredible useful, or may add a bit of functionality that you've been wanting.  Functions that are specific to certain features are not included here, as those will be on their respective feature page.
+There are a lot of hidden functions in QMK that are incredibly useful, or may add a bit of functionality that you've been wanting.  Functions that are specific to certain features are not included here, as those will be on their respective feature page.
 
 ## (OLKB) Tri Layers :id=olkb-tri-layers