Fix custom functions documation for layer_state_set*

docs/custom_quantum_functions.md

@@ -207,7 +207,7 @@ uint32_t layer_state_set_user(uint32_t state) {
   return state;
 }
 ```
-### `matrix_init_*` Function Documentation
+### `layer_state_set_*` Function Documentation
 
 * Keyboard/Revision: `void uint32_t layer_state_set_kb(uint32_t state)`
 * Keymap: `uint32_t layer_state_set_user(uint32_t state)`