fix TOC and optional parts

"implementing the BST" is not optional.

README.md

@@ -30,9 +30,9 @@ Finally, a few examples of trees will be showcased:
       - [_Question 10._](bst.md#question-10-toc)
     - [_Deletion_](bst.md#deletion-toc)
       - [_Question 11._](bst.md#question-11-toc)
-- [_Unbalanced trees_ (_optional_)](avl.md#unbalanced-trees-toc-optional)
-  - [_Question 12._](avl.md#question-12-toc)
-  - [_Height-balanced BSTs_](avl.md#height-balanced-bsts-toc)
+- [_Unbalanced trees_ (_optional_)](bst.md#unbalanced-trees-toc-optional)
+  - [_Question 12._](bst.md#question-12-toc)
+- [_Height-balanced BSTs_ (_optional_)](avl.md#height-balanced-bsts-toc-optional)
   - [_Question 13._](avl.md#question-13-toc)
   - [_Question 14._](avl.md#question-14-toc)
   - [_Question 15._](avl.md#question-15-toc)

avl.md

-# Height-balanced BSTs [[toc](README.md#table-of-content)]
+# Height-balanced BSTs [[toc](README.md#table-of-content)] (_optional_)
 
 First, let's formalize a bit the intuition from previous section about "_what it
 means for a tree to be unbalanced?_"

bst.md

-### Implementing the BST [[toc](README.md#table-of-content)] (_optional_)
+### Implementing the BST [[toc](README.md#table-of-content)]
 
 Now that we have seen what trees are and the basic operations on binary trees,
 it is time to go back to our original problem.
@@ -114,7 +114,7 @@ Well, once again, there are a few cases to look at
 :gear: Run `make test-bst`. If the "delete" test is green, good job :clap:
 :clap:
 
-## Unbalanced trees [[toc](README.md#table-of-content)]
+## Unbalanced trees [[toc](README.md#table-of-content)] (_optional_)
 Finally, we have a fully working BST! :tada:
 
 However, it is not entirely time to celebrate... We have a problem.