fix compile error

Author: jonsequitur

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -161,7 +161,7 @@ public void custom_parsing_of_sequence_value_from_an_argument_with_one_token()
             {
                 var argument = new Argument<IEnumerable<int>>(result =>
                 {
-                    result.Value = result.Tokens.Single().Value.Split(",").Select(int.Parse);
+                    result.Value = result.Tokens.Single().Value.Split(',').Select(int.Parse);
 
                     return true;
                 });