promises: fix broken URL
promises: fix broken URL

googlecodelabs/workbox-lab,googlecodelabs/amp-pwa-workbox,googlecodelabs/workbox-indexeddb,google-developer-training/amp-ilt,google-developer-training/amp-ilt,google-developer-training/pwa-training-labs,verioussmith/pwa-training-labs,verioussmith/pwa-training-labs,emoreno911/pwa-training-labs,google-developer-training/pwa-training-labs,googlecodelabs/workbox-lab,googlecodelabs/amp-pwa-workbox,googlecodelabs/workbox-indexeddb,emoreno911/pwa-training-labs