Fixed typo in description "then" to "the" (#4858)

Author: abegomez

docs/standard/using-linq.md

@@ -139,7 +139,7 @@ var cats = dogs.Select(dog => dog.TurnIntoACat());
 var queryCats = from dog in dogs
                 select dog.TurnIntoACat();
 
-// Summing then lengths of a set of strings
+// Summing the lengths of a set of strings
 int seed = 0;
 int sumOfStrings = strings.Aggregate(seed, (s1, s2) => s1.Length + s2.Length);
 ```