[fix] schema initialization id to match docs/schemata.md#meta-data spec

According to https://github.com/interagent/prmd/blob/master/docs/schemata.md#meta-data:
- `id` - an id for this schema, it MUST be in the form `"schema/#{lower_case_singular_resource}"`

Currently `prmd init` generates `schemata/XXX` instead of `schema/XXX`, behaviour corrected in this PR. 

If I am mistaken I can correct `schemata.md` and this PR should be rejected.

Thanks