Fix 'remote_write' config example (#2807)

When the contents of `remote_write` are a map, as it currently is in this README, Prometheus 1.6.3 fails to start with error message:
```
Error loading config: couldn't load configuration (-config.file=/etc/prometheus/prometheus.yml): yaml: unmarshal errors:\n  line 14: cannot unmarshal !!map into []*config.RemoteWriteConfig" source="main.go:159"
```
Changing it to a list of keyword: value pairs, fixes the error.