Update rule-checking documentation to use "promtool".
Update rule-checking documentation to use "promtool".

tcolgate/docs,Cepave/openfalcon.io,du2016/prometheus-docs,pdbogen/docs,ddfreyne/prometheus-docs,dln/docs,vic3lord/docs,chosenken/docs,jmptrader/docs,Cepave/openfalcon.io,chosenken/docs,jimmidyson/docs,open-falcon/open-falcon.github.io,vic3lord/docs,chosenken/docs,evanphx/docs,ChaoticMind/docs,evanphx/docs,jmptrader/docs,ddfreyne/prometheus-docs,open-falcon/open-falcon.github.io,du2016/prometheus-docs,dato/prometheus-docs,prometheus/docs,dln/docs,prometheus/docs,dato/prometheus-docs,chosenken/docs,jimmidyson/docs,jimmidyson/docs,tcolgate/docs,tcolgate/docs,ChaoticMind/docs,prometheus/docs,evanphx/docs,vic3lord/docs,pdbogen/docs,ddfreyne/prometheus-docs,pdbogen/docs,dln/docs,du2016/prometheus-docs,jmptrader/docs,ddfreyne/prometheus-docs,ChaoticMind/docs,Cepave/openfalcon.io,open-falcon/open-falcon.github.io,dato/prometheus-docs