linked-list: use tildes for admonition, not backticks (#2255)

A recent commit [1] added an admonition that used backticks, but every
other admonition uses tildes.

From the Exercism Markdown specification [2]:

    We support special types of blocks that can be added to documents to pull out commentary that doesn't fit with the main body of the text.

    [...]

    All blocks are written using 4 tildes, in the form of:

    ~~~~exercism/note
    Content goes here

    You can include code:
    ```ruby
    str = "Hello, World"
    ```
    ~~~~

    (Note: You may also use backticks or other levels of tildes in exceptional circumstances)

[1] d48e19a ("Rework linked-list documentation", 2023-04-09)
[2] https://github.com/exercism/docs/blob/8c604214c423/building/markdown/markdown.md#special-blocks-sometimes-called-admonitions

Author: ee7

exercises/linked-list/instructions.md

@@ -13,7 +13,7 @@ Sometimes a station gets closed down, and in that case the station needs to be r
 
 The size of a route is measured not by how far the train travels, but by how many stations it stops at.
 
-```exercism/note
+~~~~exercism/note
 The linked list is a fundamental data structure in computer science, often used in the implementation of other data structures.
 As the name suggests, it is a list of nodes that are linked together.
 It is a list of "nodes", where each node links to its neighbor or neighbors.
@@ -23,4 +23,4 @@ In a **doubly linked list** each node links to both the node that comes before,
 If you want to dig deeper into linked lists, check out [this article][intro-linked-list] that explains it using nice drawings.
 
 [intro-linked-list]: https://medium.com/basecs/whats-a-linked-list-anyway-part-1-d8b7e6508b9d
-```
+~~~~