fix comment on isHappy

src/Course/State.hs

@@ -173,7 +173,7 @@ distinct =
 -- In contrast, a sad number (not a happy number) is where the sum of the square of its digits never reaches 1
 -- because it results in a recurring sequence.
 --
--- /Tip:/ Use `findM` with `State` and `produce`.
+-- /Tip:/ Use `firstRepeat` with `produce`.
 --
 -- /Tip:/ Use `join` to write a @square@ function.
 --