Update instructions.md (#2262)

Add the missing word "at" in "Let's look at an example"

exercises/sum-of-multiples/instructions.md

@@ -14,7 +14,7 @@ The energy points are awarded according to the following rules:
 3. Remove any duplicates.
 4. Calculate the sum of all the numbers that are left.
 
-Let's look an example:
+Let's look at an example:
 
 **The player completed level 20 and found two magical items with base values of 3 and 5.**