Remaining places in draft-08 still use definitions instead of $defs

[Julian]

https://json-schema.org/work-in-progress/WIP-jsonschema-core.html#rfc.section.12.4.2

and

https://json-schema.org/work-in-progress/WIP-jsonschema-core.html#rfc.section.12.4.3

from what I found via grep.