Replace "Substring" with range syntax

Author: atifaziz

src/DocoptNet/Docopt.cs

@@ -404,7 +404,7 @@ static IEnumerable<Option> ParseShorts(Tokens tokens, ICollection<Option> option
             while (left != "")
             {
                 var shortName = "-" + left[0];
-                left = left.Substring(1);
+                left = left[1..];
                 var similar = options.Where(o => o.ShortName == shortName).ToList();
                 Option option;
                 if (similar.Count > 1)

src/DocoptNet/Extensions.cs

@@ -14,7 +14,7 @@ static class Extensions
         public static (string, string, string) Partition(this string input, string separator)
         {
             return input.IndexOf(separator, System.StringComparison.Ordinal) is var i and >= 0
-                 ? (input.Substring(0, i), separator, input.Substring(i + separator.Length))
+                 ? (input[..i], separator, input[(i + separator.Length)..])
                  : (input, string.Empty, string.Empty);
         }
     }