Convert backtick (`) admonition fences to tildes (~).

exercises/practice/sieve/.docs/instructions.md

@@ -18,11 +18,11 @@ Then you repeat the following steps:
 You keep repeating these steps until you've gone through every number in your list.
 At the end, all the unmarked numbers are prime.
 
-```exercism/note
+~~~~exercism/note
 [Wikipedia's Sieve of Eratosthenes article][eratosthenes] has a useful graphic that explains the algorithm.
 
 The tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes.
 A good first test is to check that you do not use division or remainder operations.
 
 [eratosthenes]: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
-```
+~~~~