Fix lexer for "----" (<hr/>) case

The point is that there may be another character in the string.
For example, a space or a line break.

Because we check the whole string, not the first 4 characters - we can ignore the variant where it can be "----\s" or "----\r\n" and make the wrong markup.

Alternatively: use `startsWith("----")`, but this is a bit more expensive operation.

markup/lexer.go

@@ -235,7 +235,7 @@ normalState:
 	case startsWith("<="):
 		addParagraphIfNeeded()
 		addLine(parseTransclusion(line, state.name))
-	case line == "----":
+	case line[:4] == "----":
 		addParagraphIfNeeded()
 		*ast = append(*ast, Line{id: -1, contents: "<hr/>"})
 	case MatchesImg(line):