Minor wording improvement in overview docs.
Minor wording improvement in overview docs.

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