the pb-out command line option is ignored in some cases

When I tried a variation of step 5 of the README.md, petstore.pb was not created in the current directory. See the following shell transcript (newlines added for clarity):

```
Jeff:gRPC jeff$ ls
README_install_protoc grpc
adapter-ccf_grpc grpc_poc_scaffold
adapter-grpc_poc protos
adapter-grpc_poc.tgz protos.zip
adapter-kafka protos_ccf_grpc_20210113
gnostic

Jeff:gRPC jeff$ mkdir sandbox

Jeff:gRPC jeff$ cd sandbox/

Jeff:sandbox jeff$ gnostic --pb-out=. ../gnostic/examples/v2.0/json/petstore.json

Jeff:sandbox jeff$ ls -l

Jeff:sandbox jeff$ find .. -name petstore.pb -ls
16592375 8 -rw-r--r-- 1 jeff staff 1031 Mar 10 17:40 ../gnostic/examples/v2.0/json/petstore.pb
```