Assorted updates to README.vms
Directory rename no longer needed on VMS.

As of 5.18.0, Perl on VMS can (at last) be built in and installed
from a directory having dots in the name, so it is no longer
necessary to rename the top-level source directory before building.

The pertinent instructions have been removed from README.vms, so
we also no longer need to update the version number that was
embedded in those instructions.