small doc fix

On "Collecting common values" (in formulas) with grains.filter_by.  According to [docs](https://github.com/saltstack/salt/blob/1e7bc9cba14cbf08108c7b3223c2333cc413e50f/salt/modules/grains.py#L420-L424) ``base`` arg should be a key (string) of the given dict