markdown spec: consider changing admonition syntax from tildes to backticks

[ee7]

We merged exercism/problem-specifications#2322, but the markdown spec currently says:

docs/building/markdown/markdown.md

Lines 108 to 121 in cbb4653

	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)

[ee7]

We decided to stick with tildes. Closing.