Clarify sentence when 'branch' is not set in source config

[bgandon]

Hi there,

I've come across this sentence about source.branch:

If unset for get, the repository's default branch is used;

and tought it could be much clearer:

If unset, get steps will checkout the repository's default branch;

Happy to receive feedback and help,
Benjamin

[taylorsilva]

LGTM, thanks for the PR!