[Crystal] use encode_path instead of encode (deprecated)

In src/petstore/api/pet_api.cr:277:68

 277 | local_var_path = "/pet/{petId}".sub("{" + "petId" + "}", URI.encode(pet_id.to_s).gsub("%2F", "/"))
                                                                    ^-----
Warning: Deprecated URI.encode. Use `.encode_path` instead.

Address the above warning

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cc @cyangle