fix: unclosed headings in html

src/markdown.rs

@@ -217,7 +217,7 @@ fn render_markdown<'md>(src: &'md str, mut p: ParserVars<'md>) -> std::io::Resul
                             p.links.clear();
                         }
                     }
-                    Heading(level) => p.html.write_fmt(format_args!("<{}>", level))?,
+                    Heading(level) => p.html.write_fmt(format_args!("</{}>", level))?,
                     BlockQuote(_) => p.html.write_all(b"</blockquote>")?,
                     CodeBlock => {
                         p.html.write_all(b"</code></pre>")?;