Use vtypei instead of zimm in vcfg-format.adoc (#862)

This matches how the vset(i)vli instructions are defined.

I'm not sure what the 'z' in zimm was meant to convey. The base ISA spec uses nzimm, but to mean an immediate that can't be zero, but I don't think it ever uses zimm.

Signed-off-by: Craig Topper <craig.topper@gmail.com>