Revert SchemaGenerator description value from tuple to str

[arttuperala]

Description of the Change

Fixes a schema issue introduced in 9ced3be that would result in broken YAML schema.

components:
  schemas:
    error:
      properties:
        source:
          properties:
            pointer:
              description: !!python/tuple
              - A [JSON Pointer](https://tools.ietf.org/html/rfc6901) to the associated
                entity in the request document [e.g. `/data` for a primary data object,
                or `/data/attributes/title` for a specific attribute.

Only present on the main branch so I didn't update the changelog.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[sliverc]

Great catch. I am confused though that our test_openapi.py did not catch this. Could you have a brief look at the test and see whether it is easy to adjust it so it covers this part of the schema as well?

[arttuperala]

Converted the existing schema construction test to a snapshot one. Do you think that'll suffice?

[sliverc]

Awesome. Thanks, Yep, makes most sense to have this schema test as a snapshot test. Like this, it would have caught this error.