Fix label name in query language example.
Fix label name in query language example.

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