Fix spelling error and heading in instruction.append file. (#3807)

Author: BethanyG

exercises/practice/linked-list/.docs/instructions.append.md

@@ -5,7 +5,7 @@
 While linked lists can be implemented in a variety of ways with a variety of underlying data structures, we ask here that you implement your linked list in an OOP fashion.
 
 In the stub file, you will see the start of a `Node` class, as well as a `LinkedList` class.
-Your `Node` class should keep track of its value, as well as which other nodes preceed or follow.
+Your `Node` class should keep track of its value, as well as which nodes precede or follow.
 Your `push`, `pop`, `shift`, `unshift`, and the special method for `len` should be implemented in the `LinkedList` class.
 You might also find it useful to implement a special `iter` method for iteration.
 
@@ -17,7 +17,7 @@ If the value appears more than once, only the **first** occurrence should be rem
 
 <br>
 
-## Exception messages
+## Exception Messages
 
 Sometimes it is necessary to [raise an exception][raising]. When you do this, you should always include a **meaningful error message** to indicate what the source of the error is.
 This makes your code more readable and helps significantly with debugging.