Escape unrecognized identifiers as they may contain '<' or '>'.

src/why3doc/doc_lexer.mll

@@ -74,7 +74,7 @@
               Doc_def.pp_locate id Pp.html_string s
       with Not_found ->
         (* otherwise, just print it *)
-        pp_print_string fmt s
+        Pp.html_string fmt s
     end
 
   type empty_line = PrevEmpty | CurrEmpty | NotEmpty