chore: small typo in variables challenge (#25)

public/lessons/variables.cairo

@@ -21,7 +21,7 @@
 //    and run the code again.
 //
 // The behavior of the code doesn't change. This is because
-// the keyword `local` allocates a new variables for the
+// the keyword `local` allocates a new variable for the
 // intermediate values as well. The main difference between
 // `local` and `tempvar` is explained in the next lesson.