three suggested changes to the starter code for the Junit studio code

[chats-perches]

1. moving the long comment from BalancedBrackets.java to BalancedBracketsTest.java.
2. adding to the long comment the instruction that it's best for this studio to write the tests first before looking at the code in BalancedBrackets.java
3. removing the examples of test strings from the long comment.