Do not match headlines as plain list items.

src/parser/plain_list.rs

@@ -102,6 +102,22 @@ pub(crate) fn detect_not_plain_list_item_indent<'b, 'g, 'r, 's>(
     {
         return Ok((input, indent));
     }
+
+    // Headlines are not plain list items.
+    if let Ok((_remaining, (_, indent, _))) = verify(
+        tuple((
+            start_of_line,
+            parser_with_context!(indentation_level)(context),
+            tuple((
+                parser_with_context!(bullet)(context),
+                alt((space1, line_ending, eof)),
+            )),
+        )),
+        |(_, (depth, bullet), (_, _))| *depth == 0 && Into::<&str>::into(bullet).starts_with('*'),
+    )(input)
+    {
+        return Ok((input, indent));
+    }
     return Err(nom::Err::Error(CustomError::Static("No element detected.")));
 }