openvms: fix example name

Commit efc696a2

 renamed persistant.c to persistent.c to
fix the typo in the name, but missed to update the OpenVMS package
files which still looked for the old name.

Closes #3217
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Viktor Szakats <commit@vszakats.net>