fixing link in hints (#2099)

Author: michalporeba

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

@@ -1,4 +1,4 @@
 # Hints
 
 This exercise requires you to create a linked list data structure which can be iterated. This requires you to implement the IEnumerable\<T> interface.
-For more information, see [this page](https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1
+For more information, see [this page](https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1).