fix(docs): Update CONTRIBUTING.md to reflect build process (#276)

* fix(docs): Update CONTRIBUTING.md to reflect build process

This updated the CONTRIBUTING.md file to have the proper command tok
build and run `stackdriver_exporter`.

Signed-off-by: pokom <mark.poko@grafana.com>

* Update CONTRIBUTING.md

Signed-off-by: Ben Kochie <superq@gmail.com>

---------

Signed-off-by: pokom <mark.poko@grafana.com>
Signed-off-by: Ben Kochie <superq@gmail.com>
Co-authored-by: Ben Kochie <superq@gmail.com>

Author: Pokom

CONTRIBUTING.md

@@ -33,8 +33,8 @@ For complete instructions on how to compile see: [Building From Source](https://
 For quickly compiling and testing your changes do:
 ```
 # For building.
-go build ./cmd/prometheus/
-./prometheus
+make build
+./stackdriver_exporter
 
 # For testing.
 make test         # Make sure all the tests pass before you commit and push :)