Remove unnecessary "ref" modifier on "ParseResult<>" (#96)

Co-authored-by: Sébastien Ros <sebastienros@gmail.com>

Author: atifaziz

src/Parlot/Fluent/ParseResult.cs

@@ -1,6 +1,6 @@
 namespace Parlot
 {
-    public ref struct ParseResult<T>
+    public struct ParseResult<T>
     {
         public ParseResult(int start, int end, T value)
         {