Fix small typo in introduction (#2520)

s/lookup/look up/

Author: blakelewis

exercises/pascals-triangle/introduction.md

@@ -13,7 +13,7 @@ Over the next hour, your teacher reveals some amazing things hidden in this tria
 - It contains the Fibonacci sequence.
 - If you color odd and even numbers differently, you get a beautiful pattern called the [Sierpiński triangle][wikipedia-sierpinski-triangle].
 
-The teacher implores you and your classmates to lookup other uses, and assures you that there are lots more!
+The teacher implores you and your classmates to look up other uses, and assures you that there are lots more!
 At that moment, the school bell rings.
 You realize that for the past hour, you were completely absorbed in learning about Pascal's triangle.
 You quickly grab your laptop from your bag and go outside, ready to enjoy both the sunshine _and_ the wonders of Pascal's triangle.