Setting up config for the first time gets misleading warning

[jennli]

Logging a tiny usability bug:

Getting the above output when I was setting the config. Specifically

The default filename has changed. You may need to rename
vsts-runner-config.json
to
azure-devops-runner-config.json

That warning msg will show up for everybody who's setting up the config for the first time, since azure-devops-runner-config.json has yet been created, which is misleading.