fix: ascii start

Signed-off-by: Sachin Beniwal <s474996633@gmail.com>

Author: benodiwal

src/dune_sexp/lexer.mll

@@ -182,7 +182,7 @@ and atom acc start = parse
     { atom ((template_variable lexbuf) :: acc) start lexbuf }
   | "%"
     { atom (Template.add_text acc "%") start lexbuf }
-  | ['\127'-'\255']
+  | ['\128'-'\255']
     { error lexbuf Atom.non_ascii_error_message }
   | ""
     { Template.token acc ~quoted:false ~start lexbuf }