Fix comment typo in instructions.md (#885)

typo

Author: yura

exercises/concept/expert-mixologist/.docs/instructions.md

@@ -73,5 +73,5 @@ let orders = [
 
 orderTracker(orders: orders)
 
-// returnsa (beer: (first "10:01", last "10:22", total 3), soda: (first "10:09", last "10:09", total 1))
+// returns (beer: (first "10:01", last "10:22", total 3), soda: (first "10:09", last "10:09", total 1))
 ```