Recommended edits to invoice-processing README

[seanofahey]

Thanks for the great repo and accompanying video. In running through the invoice-processing-pipeline README, though, I found that I had to make two changes to make the job run successfully. I wanted to share them along with one additional recommendation:

1. after initiating the Firestore database API, I still needed to go to the Firestore webpage to select between the native and datastore modes before the api would function. Perhaps you should add a bullet under the * Navigate to the Document AI section (also note the typo with 'the the')
2. there is a missing '' in the second command in the "Create the Cloud Run Job" section. It should be --image gcr.io/$GOOGLE_CLOUD_PROJECT/invoice-processor \ otherwise the env variables aren't passed
3. (recommendation) making the GCP region an env variable vs. hardcoded would make it easier to run this code in different parts of the world without typing over 'europe-west9'