fix misspelling of "thematic" causing render errors

[jonklein]

Currently in master:

iex(9)> MDEx.parse_document!("---\nTest") |> MDEx.to_html
{:error,
 {:unknown_node_name, "themantic_break", "(<<\"themantic_break\">>, [], [])"}}

The markdown --- corresponds to a "thematic break" which should produce a <hr/> HTML tag, but a typo ("themantic") in the name causes an error when rendering the content.

This PR fixes the name and adds a test.