Use cpu metric for CPU-based autoscaling sample (#1918)

Using `autoscaling.knative.dev/metric: concurrency`
made no sense to me so I'm doing a quick PR.

An example of this can be found at https://medium.com/@mchmarny_google/https-medium-com-p-knative-autoscaling-love-story-e32a27b7855
but let's not try to send people to external links to
find how to do this.

Author: ahmetb

docs/serving/configuring-the-autoscaler.md

@@ -180,10 +180,12 @@ spec:
   template:
     metadata:
       annotations:
-        autoscaling.knative.dev/metric: concurrency
+        autoscaling.knative.dev/metric: cpu
+        autoscaling.knative.dev/target: 70
         autoscaling.knative.dev/class: hpa.autoscaling.knative.dev
 ```
 
+
 ## Additional resources
 
 - [Go autoscaling sample](https://knative.dev/docs/serving/samples/autoscale-go/index.html)