Update metrics-graph flag description

Useful to more people to give an example of usage with regular
Prometheus, rather than the style which is used with the Weave Cloud
service.

prog/main.go

@@ -389,7 +389,7 @@ func setupFlags(flags *flags) {
 	flag.IntVar(&flags.app.memcachedCompressionLevel, "app.memcached.compression", gzip.DefaultCompression, "How much to compress reports stored in memcached.")
 	flag.StringVar(&flags.app.userIDHeader, "app.userid.header", "", "HTTP header to use as userid")
 	flag.BoolVar(&flags.app.externalUI, "app.externalUI", false, "Point to externally hosted static UI assets")
-	flag.StringVar(&flags.app.metricsGraphURL, "app.metrics-graph", "", "Enable extended metrics graph by providing a templated URL (supports :instanceID and :query). Example: --app.metrics-graph=/prom/:instanceID/notebook/new")
+	flag.StringVar(&flags.app.metricsGraphURL, "app.metrics-graph", "", "Enable extended metrics graph by providing a templated URL. Example: --app.metrics-graph=http://localhost:9090/graph?g0.expr=:query&g0.tab=0")
 	flag.StringVar(&flags.app.serviceName, "app.service-name", "app", "The name for this service which should be reported in instrumentation")
 
 	flag.IntVar(&flags.app.blockProfileRate, "app.block.profile.rate", 0, "If more than 0, enable block profiling. The profiler aims to sample an average of one blocking event per rate nanoseconds spent blocked.")