Update app-plugins.md (#445)
Update app-plugins.md (#445)

"plugin" to "plugins" in example
rstoenescu/quasar-site,quasarframework/quasar-framework.org,quasarframework/quasar-framework.org,rstoenescu/quasar-site,rstoenescu/quasar-site,quasarframework/quasar-framework.org