Corrected service-account-key.json

Updated Readme to correct service_account_key.json file name to service-account-key.json since the former causes an exception while uploading videos
{ Error: ENOENT: no such file or directory, open '/srv/lib/service-account-key.json'
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/srv/lib/service-account-key.json' }

Hence found that the correct file name should be service-account-key.json as opposed to what was mentioned in the readme.
Also this caused other issues since the video was not converted, but still the number of images was updated and showed incorrect numbers.

Author: ganeshsp1

README.md

@@ -74,7 +74,7 @@ $ firebase functions:config:set sendgrid.key="<API_KEY>"
 5. Next, set the `FROM_EMAIL` constant in [this](functions/src/constants.ts) file to be email ID from which emails will be sent.
 6. Follow that up with setting the `APP_NAME` that you've decided for your app.
 7. Go to the Firebase Console, select the gear image > **project settings** > **Service Accounts** and click **Generate New Private Key** to download a Service Account Key JSON document.
-8. Add the JSON service account credentials file to the **Functions** directory as **functions/lib/service_account_key.json**.
+8. Add the JSON service account credentials file to the **Functions** directory as **functions/lib/service-account-key.json**.
 9. Now run, `npm run deploy` to deploy the functions, firestore & storage configuration.
 
 **Flutter app setup**