Update example run line in README.md to match updated argument names/requirements

[SDParish]

Currently says:

php example.php --yaml-path /path/to/api.yaml --model-dir /path/to/generate/models --namespace MyNamespace

However the --model-dir argument flag has been renamed, and a --more-specificity argument is now required.
I believe it should now read:

php example.php --yaml-path /path/to/api.yaml --dir /path/to/generate/models --namespace MyNamespace --more-specificity true