Fix SchemaRegistryClient docs

[pko-trackunit]

Fixed few mismached URLs and missing default value for a parameter.

[pko-trackunit]

Also, it feels like lookup_schema() method should not be using an endpoint that "Register a new schema under the specified subject". Unless I am misunderstanding what "lookup" means.

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[fangnx]

Hi @pko-trackunit, thanks for raising the PR. I double checked and the two doc URLs you fixed and the one for lookup_schema() (should be https://docs.confluent.io/platform/current/schema-registry/develop/api.html#post--subjects-(string- subject)) are indeed incorrect

Also, it feels like lookup_schema() method should not be using an endpoint that "Register a new schema under the specified subject". Unless I am misunderstanding what "lookup" means.

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[rayokota]

/sem-approve