Update lib/src/arg_parser_exception.dart

Co-authored-by: Nate Bosch <[email protected]>

lib/src/arg_parser_exception.dart

@@ -9,7 +9,7 @@ class ArgParserException extends FormatException {
   /// This will be empty if the error was on the root parser.
   final List<String> commands;
 
-  /// The argument that were being parsed when the error was discovered.
+  /// The argument name that was being parsed when the error was discovered.
   final String? arg;
 
   ArgParserException(super.message,