Fix code references in comments.

petitparser/lib/src/core/parser.dart

@@ -63,11 +63,11 @@ abstract class Parser<T> implements Pattern {
   /// the internal parsing logic of the receiving parser.
   ///
   /// For example, `letter().plus().parse('abc')` results in an instance of
-  /// [Success], where [Result.position] is `3` and [Success.value] is
+  /// [Success], where [Context.position] is `3` and [Success.value] is
   /// `[a, b, c]`.
   ///
   /// Similarly, `letter().plus().parse('123')` results in an instance of
-  /// [Failure], where [Result.position] is `0` and [Failure.message] is
+  /// [Failure], where [Context.position] is `0` and [Failure.message] is
   /// ['letter expected'].
   Result<T> parse(String input) => parseOn(Context(input, 0));