Fix fiber link

exercises/practice/parallel-letter-frequency/.docs/instructions.append.md

@@ -2,5 +2,5 @@
 ## Further Reading
 - [Ruby `Thread` Documentation](https://docs.ruby-lang.org/en/master/Thread.html)
 - [Ruby `Thread::Queue` Documentation](https://docs.ruby-lang.org/en/master/Thread/Queue.html)
-- [Ruby `Fiber` Documentation](https://docs.ruby-lang.org/en/master/Thread.html)
+- [Ruby `Fiber` Documentation](https://docs.ruby-lang.org/en/master/Fiber.html)
 - [Ruby `Ractor` Documentation](https://docs.ruby-lang.org/en/master/Ractor.html)