Format all instructions.append.md to start with an H1 (required but ignored) and H2

exercises/practice/anagram/.docs/instructions.append.md

@@ -1,3 +1,5 @@
 # Instructions Append
 
+## Implementation
+
 You must return the anagrams in the same order as they are listed in the candidate words.

exercises/practice/nucleotide-count/.docs/instructions.append.md

@@ -1,5 +1,7 @@
 # Instructions append
 
+## Implementation
+
 There are times when you need to handle unexpected issues that arise during code execution.
 In Dart, you can use exceptions to handle these situations.
 

exercises/practice/phone-number/.docs/instructions.append.md

@@ -1,4 +1,6 @@
-# Tip
+# Instructions append
+
+## Tip
 
 A function with a *return type* can only return data of that *type* and `null`.
 However the function caller is only expecting one data type.