Fix code style

Author: Witiko

markdown.dtx

@@ -27331,7 +27331,8 @@ function M.writer.new(options)
 
       -- Remove everything up to the first non-space.
       if not letter_found then
-        local is_letter = not lpeg.match(parsers.unicode.whitespace, char)
+        local is_letter
+          = not lpeg.match(parsers.unicode.whitespace, char)
         if is_letter then
           letter_found = true
         else