exercises: docs: change admonitions to use 4 tildes again

[ee7]

Commit 3eed72b changed admonitions to use 3 backticks while reflecting a commit in problem-specifications, but problem-specfications is reverting that change to admonitions (coming in exercism/problem-specifications#2327).

---

Commit the result of running:

git ls-files 'exercises/practice/*/.docs/*.md' | xargs sed -i 's_```exercism/_~~~~exercism/_g'

With this PR, there are no more backtick admonitions in this repo:

$ git rev-parse --short HEAD
1878722
$ git grep '```exercism' || echo none
none

[ErikSchierboom]

I will bulk PR this too, but you're quicker than I am :)