Remove deleted npm scripts from `contributing.md` (#492)

Description of changes

Removes npm `test` and `build:docs` scripts from `contributing.md`, which no longer exist.

CONTRIBUTING.md

@@ -55,9 +55,7 @@ When you're done making changes, check that they pass linting, formatting, tests
 ```
 npm run lint
 npm run fmt
-npm run test
 npm run build
-npm run build:docs
 ```
 
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