Update instructions for parallel-letter-frequency (#2713)

exercises/practice/parallel-letter-frequency/.docs/hints.md

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+# Hints
+
+The following resources may help you with this:
+
+- [Concurrency (Oracle Java Tutorials)](https://docs.oracle.com/javase/tutorial/essential/concurrency)
+- [Java Concurrency (Baeldung)](https://www.baeldung.com/java-concurrency)
+- [Java 8 Parallel Streams (Baeldung)](https://www.baeldung.com/java-8-streams#parallel-streams)

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

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 # Instructions append
 
-Single-threaded (non-concurrent) solutions can pass all tests [but the last.](https://www.youtube.com/watch?v=mJZZNHekEQw)  Your solution will be tested for concurrency by submitting it as a [Runnable](https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html) to an [ExecutorService.](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html) Your solution must leverage multiple [Threads](https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html) to pass the final test.
-
-Java documentation on [parallel streams](https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html) may provide some help.
-
 As a stretch goal, consider if your implementation will work for characters with [diacritics or accents](https://en.wikipedia.org/wiki/Diacritic). For example, such solutions should not consider e and ë the same character. An example text for this case is [Wilhelmus](https://en.wikipedia.org/wiki/Wilhelmus), the Dutch national anthem.