style(cd): use set for the true value of boolean variables

Author: akinomyoga

completions/cd

@@ -25,8 +25,8 @@ _comp_cmd_cd()
     fi
 
     local mark_dirs="" mark_symdirs=""
-    _comp_readline_variable_on mark-directories && mark_dirs=y
-    _comp_readline_variable_on mark-symlinked-directories && mark_symdirs=y
+    _comp_readline_variable_on mark-directories && mark_dirs=set
+    _comp_readline_variable_on mark-symlinked-directories && mark_symdirs=set
 
     # we have a CDPATH, so loop on its contents
     local paths dirs