Update documentation links from referencing 3.0.2 to 3.1.0

[kdipippo]

This repo's spec is in OpenAPI 3.1.0:

openapi-starter/openapi/openapi.yaml

Line 1 in d6c17ac

openapi: 3.1.0

But the links here are to the 3.0.2 markdown file when 3.1.0.md exists:

openapi-starter/openapi/openapi.yaml

Line 22 in d6c17ac

format](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md).

It may be enough to change the links and be done with it, but I am unsure if there's also other parts of this template that need a 3.0.2 -> 3.1.0 upgrade.

[kdipippo]

Created #118

EDIT: I accidentally closed this PR. So resolving this issue means replicating the work I performed in there.