Fix small grammatical issues w/ AlertManager docs
Fix small grammatical issues w/ AlertManager docs

tcolgate/docs,ddfreyne/prometheus-docs,tcolgate/docs,tcolgate/docs,du2016/prometheus-docs,open-falcon/open-falcon.github.io,du2016/prometheus-docs,chosenken/docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,ddfreyne/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,Cepave/openfalcon.io,Cepave/openfalcon.io,open-falcon/open-falcon.github.io,chosenken/docs,prometheus/docs,chosenken/docs,chosenken/docs,open-falcon/open-falcon.github.io,Cepave/openfalcon.io