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

exercises/practice/gigasecond/.docs/introduction.md

@@ -13,12 +13,12 @@ Then we can use metric system prefixes for writing large numbers of seconds in m
 - Perhaps you and your family would travel to somewhere exotic for two megaseconds (that's two million seconds).
 - And if you and your spouse were married for _a thousand million_ seconds, you would celebrate your one gigasecond anniversary.
 
-```exercism/note
+~~~~exercism/note
 If we ever colonize Mars or some other planet, measuring time is going to get even messier.
 If someone says "year" do they mean a year on Earth or a year on Mars?
 
 The idea for this exercise came from the science fiction novel ["A Deepness in the Sky"][vinge-novel] by author Vernor Vinge.
 In it the author uses the metric system as the basis for time measurements.
 
 [vinge-novel]: https://www.tor.com/2017/08/03/science-fiction-with-something-for-everyone-a-deepness-in-the-sky-by-vernor-vinge/
-```
+~~~~

exercises/practice/pangram/.docs/introduction.md

@@ -7,10 +7,10 @@ To give a comprehensive sense of the font, the random sentences should use **all
 They're running a competition to get suggestions for sentences that they can use.
 You're in charge of checking the submissions to see if they are valid.
 
-```exercism/note
+~~~~exercism/note
 Pangram comes from Greek, παν γράμμα, pan gramma, which means "every letter".
 
 The best known English pangram is:
 
 > The quick brown fox jumps over the lazy dog.
-```
+~~~~

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
-```
+~~~~