Fix: update package.json scripts for npm users (carbon-design-system#…

…1103)

* Fix: update package.json scripts for npm users

The script for `dev:clean` is currently `yarn build && yarn clean` or something like this. 

If a user is utilizing npm as a package manager instead of yarn, or don't have yarn installed on their machine, they will continue to see error when running `npm run dev:clean`.

* fix(package.json): update script to support npm

packages/example/package.json

@@ -5,7 +5,7 @@
   "license": "Apache 2.0",
   "scripts": {
     "dev": "gatsby develop -H 0.0.0.0",
-    "dev:clean": "yarn clean && yarn dev",
+    "dev:clean": "gatsby clean && gatsby develop -H 0.0.0.0",
     "build": "gatsby build",
     "build:clean": "yarn clean && gatsby build",
     "serve": "gatsby serve",