fixing fenced practice challenge

Author: jt14den

episodes/exercise.md

@@ -82,13 +82,15 @@ That's it! Using those four key steps, we have learned the basics of computation
 
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-:::challenge
+::::::::::::::::::::::::::::::::: challenge
+
 ## Practice
 
 Use the algorithm above to add up all the numbers between 1 and 24, 1 and 50, and 1 and 1,000.
-:::
 
-:::: discussion
+::::::::::::::
+
+::::::::::::::::::::::::::::::::: discussion
 
 ## Discussion