Fix inconsistent usage in examples (fixes #21)

[clemensv]

Summary

This PR fixes the inconsistent usage of $ref in examples throughout the spec, ensuring all examples comply with the rule stated in Section 3.4.1.

Changes

Updated examples to consistently use $ref only inside the ype attribute:

1. Set items example (line 580): Changed from bare $ref to { "type": { "": ... } }
2. Map values example (line 606): Changed from bare $ref to { "type": { "": ... } }
3. Choice choices examples (lines 738-739): Changed from bare $ref to { "type": { "": ... } }

Rationale

Section 3.4.1 explicitly states:

"The keyword is only permitted inside the 'type' attribute value of a schema definition, including in type unions."
" is NOT permitted in other attributes..."

These changes ensure all examples follow this rule consistently.

Fixes #21