redo the section readmes

exercises/tests/README.md

@@ -1,3 +1,7 @@
+### Tests
+
 Going out of order from the book to cover tests -- many of the following exercises will ask you to make tests pass!
 
-For this exercise check out the section [How to Write Tests](https://doc.rust-lang.org/book/2018-edition/ch11-01-writing-tests.html) of the Rust Book.
+#### Book Sections
+
+- [Writing Tests](https://doc.rust-lang.org/stable/book/ch11-01-writing-tests.html)