Fix example docker command line

Created by: jeanlucmongrain

Problem:

cd ./modules/openapi-generator/src/test/resources/3_0/
docker run --rm \
> -v ${PWD}:/local openapitools/openapi-generator-cli generate \
> -i petstore.yaml \
> -g go \
> -o /local/out/go
[error] The spec file is not found: petstore.yaml
[error] Check the path of the OpenAPI spec and try again.

Solution: specify absolute path to input file