Fixed typo

docs/custom_quantum_functions.md

@@ -36,7 +36,7 @@ enum my_keycodes {
 
 ## Programming the Behavior of Any Keycode
 
-When you want to override the behavior of an existing key, or define the behavior for a new key, you should use the `process_record_kb()` and `process_record_user()` functions. These are called by QMK during key processing before the actual key event is handled. If these functions return `true` QMK will process the keycodes as usual. That can be handy for extending the functionality of a key rather than replacing it. If these functions return `false` QMK will skip the normal key handling, and it will be up you to send any key up or down events that are required.
+When you want to override the behavior of an existing key, or define the behavior for a new key, you should use the `process_record_kb()` and `process_record_user()` functions. These are called by QMK during key processing before the actual key event is handled. If these functions return `true` QMK will process the keycodes as usual. That can be handy for extending the functionality of a key rather than replacing it. If these functions return `false` QMK will skip the normal key handling, and it will be up to you to send any key up or down events that are required.
 
 These function are called every time a key is pressed or released.