Update factcheck.md
Update factcheck.md

Add instructions for setting `RATES_QUERY_DATE` on Heroku.

alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers