fix README
fix README

not `make e2e-tests` but `make e2e-test`
mmerrill3/prometheus-operator,coreos/prometheus-operator,jescarri/prometheus-operator,mmerrill3/prometheus-operator,brancz/prometheus-operator,brancz/prometheus-operator,brancz/prometheus-operator,mmerrill3/prometheus-operator,gianrubio/prometheus-operator,gianrubio/prometheus-operator,gianrubio/prometheus-operator,coreos/prometheus-operator,jescarri/prometheus-operator