docs(alert): add import in usage example (#10434)
docs(alert): add import in usage example (#10434)

add : `import { AlertController } from 'ionic-angular';` in usage section
driftyco/ionic,alan-agius4/ionic,driftyco/ionic,alan-agius4/ionic,joshesp/ionic,driftyco/ionic,driftyco/ionic,joshesp/ionic,alan-agius4/ionic,alan-agius4/ionic,TillaTheHun0/ionic,alan-agius4/ionic,TillaTheHun0/ionic,alan-agius4/ionic,joshesp/ionic,TillaTheHun0/ionic,joshesp/ionic,TillaTheHun0/ionic,driftyco/ionic