Add ref instead all of for additionalProperties lesson

[Girishbari]

What kind of change does this PR introduce?

Refactoring Chapter 7, refactored the code to use $ref instead $allOf for explanation

Issue Number:

• Closes Use $ref instead of allOf for Extending Closed Schemas #65

Screenshots/videos:

If relevant, did you update the documentation?
No

Summary

Does this PR introduce a breaking change?
NO

[jdesrosiers]

Looks good. I just had a few editorial suggestions.

[jdesrosiers]

Suggested change

	Previously in the Objects , we learned about `additionalProperties`. However, it is important to note that `additionalProperties` only recognizes properties declared in the same [subschema](https://json-schema.org/learn/glossary#subschema) as itself.
	Previously in the Objects module, we learned about `additionalProperties`. However, it is important to note that `additionalProperties` only recognizes properties declared in the same [subschema](https://json-schema.org/learn/glossary#subschema) as itself.

[jdesrosiers]

Suggested change

	So, `additionalProperties` can restrict you from "extending" a schema using combining keywords such as `$ref`. In the following example, we can see how the `additionalProperties` can cause attempts to extend the address schema example to fail.
	So, `additionalProperties` can restrict you from "extending" a schema using combining [keywords](https://json-schema.org/learn/glossary#keyword) such as `$ref`. In the following example, we can see how `additionalProperties` can cause attempts to extend the address schema example to fail.

[jdesrosiers]

This schema will reject valid data [...]

That statement doesn't make sense to me. If the schema rejects it, it isn't valid. Maybe it needs to say something like, it rejects data you intended to be valid.