fix bug in map: it is supposed to be the index of the next line, not the current line.

[williamstein]

See, e.g., other plugins such as https://github.com/markdown-it/markdown-it-container/blob/master/index.js. I was working around this in my code. It's the sort of thing you might not notice if you're just processing markdown to create html. However, I use markdown-it only to tokenize markdown, then keep track of what markdown produced the given tokens for the purposes of caching, and then it is critically important that the map be correct.

[goessner]

Thanks for fixing. Fix was inserted manually with Version 1.0