Update usage example

[vbuberen]

This is a small change in example in README which should address 2 issues:

1. It feels a bit awkward compared to other actions to see uses via such docker... declaration. So I have changed action declaration to match usual way of declaring such things.
2. When used via docker.. syntax it is not possible to use in organisations that have policy of using only verified actions (like the one where I am currently working) as such workflow runs are canceled right away. But when I declare the job using github/stale-repos@v1.4.0 no policy issues appear and workflows work fine without any policy violations.

[zkoppert]

Thanks for taking the time to contribute this change!