Replace sample-controller with net-istio in CONTRIBUTING.md. (#407)

This patch makes a tiny change to replace `sample-controller` with
`net-istio` in CONTRIBUTING.md.

Author: nak3

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contribution guidelines
 
-So you want to hack on Knative `sample-controller`? Yay! Please refer to
+So you want to hack on Knative `net-istio`? Yay! Please refer to
 Knative's overall
 [contribution guidelines](https://www.knative.dev/contributing/) to find out how
 you can help.