Update docs/api/covidcast-signals/safegraph.md
Update docs/api/covidcast-signals/safegraph.md

Co-authored-by: Akvannortwick <fae6d2050ef16d7439ba48e8e82bea122dabfaf1@users.noreply.github.com>
cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata