Syntax reference: no mention of tilde for code block fence

[hellux]

The syntax reference says a code block may start with a fence of 3 or more backticks. However, the implementation allows tildes to be used as well, which is not mentioned in the reference.

[matklad]

I think this might be a dupe of #41.

My TL;DR: we indeed support both, it's not clear whether we should though. backtick is standard, but tilda looks nicer, but backtick is easier to type.

[hellux]

Ah, yes, seems to be a duplicate, forgot to make a search first.