Fix CONTRIBUTING documentation
Fix CONTRIBUTING documentation

'yeoman' is deprecated, use 'yo' instead

see http://yeoman.io/migrate.html

ngx-rocket/generator-ngx-rocket,ngx-rocket/generator-ngx-rocket,angular-starter-kit/generator-ngx-app,angular-starter-kit/generator-ngx-app,angular-starter-kit/generator-ngx-app,ngx-rocket/generator-ngx-rocket,ngx-rocket/generator-ngx-rocket,angular-starter-kit/generator-ngx-app