fix(gatsby-dev-cli): fix set-path-to-repo command after yargs bump

[pieh]

We were misusing .nargs function previously but it worked, but recent yargs bump cause it to fail now.

https://github.com/yargs/yargs/blob/master/docs/api.md#nargskey-count

The number of arguments that should be consumed after a key. This can be a useful hint to prevent parsing ambiguity.

set-path-to-repo requires one argument, otherwise it turns into boolean switch