Remove npm2yarn in contributing doc page (#515)

Author: benjie

website/docs/contributing.md

@@ -11,16 +11,16 @@ time!
 
 ## Development
 
-```sh npm2yarn
-npm install
-npm run watch
+```sh
+yarn install
+yarn run watch
 ```
 
 In another terminal:
 
-```sh npm2yarn
+```sh
 createdb graphile_worker_test
-npm test
+yarn test
 ```
 
 ### Using Docker to develop this module
@@ -68,6 +68,6 @@ docker run -e POSTGRES_HOST_AUTH_METHOD=trust -d -p 5432:5432 postgres:12
 
 then run
 
-```sh npm2yarn
-PGUSER=postgres PGHOST=localhost npm run db:dump
+```sh
+PGUSER=postgres PGHOST=localhost yarn run db:dump
 ```