correct a typo (kmonad#581)

This is the most formally correct pull-request I have ever laid my eyes on :)

keymap/tutorial.kbd

@@ -165,7 +165,7 @@
  ;; input (iokit-name "my-keyboard-product-string")
  ;; output (kext)
 
- ;; Comment this is you want unhandled events not to be emitted
+ ;; Comment this if you want unhandled events not to be emitted
  fallthrough true
 
  ;; Set this to false to disable any command-execution in KMonad