Fix openapi spec in quickstart doc

[psegedy]

Using openapi spec from quickstart results in

connexion.exceptions.InvalidSpecification: {'content': {'text/plain': {'schema': {'type': 'string'}}}} is not valid under any of the given schemas

Failed validating 'oneOf' in schema['properties']['paths']['patternProperties']['^\\/']['patternProperties']['^(get|put|post|delete|options|head|patch|trace)$']['properties']['responses']['patternProperties']['^[1-5](?:\\d{2}|XX)$']:
    {'oneOf': [{'$ref': '#/definitions/Response'},
               {'$ref': '#/definitions/Reference'}]}

On instance['paths']['/greeting/{name}']['get']['responses']['200']:
    {'content': {'text/plain': {'schema': {'type': 'string'}}}}  

because description is missing in the spec and it is a required property for #/definitions/Response. https://github.com/spec-first/connexion/blob/main/connexion/resources/schemas/v3.0/schema.json#L596-L602

[Ruwann]

Thanks for the contribution, @psegedy !

[coveralls]

coverage: 94.197%. remained the same
when pulling 2f2f5cb on psegedy:patch-1
into fcfe151 on spec-first:main.