Fix typo in README

The collection is called `oauth_clients`, not `oauth_client`.

As pointed out by @bdaneshgar in #40.

Author: maxbbraun

README.md

@@ -35,7 +35,7 @@ The database backend uses [Cloud Firestore](https://firebase.google.com/products
 To populate the cross-user data after [setting up](https://firebase.google.com/docs/firestore/quickstart):
 1. Obtain an API key for [Google Maps](https://cloud.google.com/maps-platform) and add it the `api_keys` collection under `/api_keys/google_maps/api_key`. Ensure that the [Maps Static API](https://console.cloud.google.com/apis/library/static-maps-backend.googleapis.com), [Directions API](https://console.cloud.google.com/apis/library/directions-backend.googleapis.com), [Geocoding API](https://console.cloud.google.com/apis/library/geocoding-backend.googleapis.com), [Maps Elevation API](https://console.cloud.google.com/apis/library/elevation-backend.googleapis.com), and [Time Zone API](https://console.cloud.google.com/apis/library/timezone-backend.googleapis.com) are all enabled and added to the key's restrictions. The [Cloud Vision API](https://console.cloud.google.com/apis/library/vision.googleapis.com) and [Google Calendar API](https://console.cloud.google.com/apis/library/calendar-json.googleapis.com) also need to be enabled, but they do not need a key.
 2. Obtain an API key for [OpenWeather](https://openweathermap.org/guide) and add it in the `api_keys` collection under `/api_keys/open_weather/api_key`.
-3. Obtain an [OAuth client ID](https://console.developers.google.com/apis/credentials) for the [Google Calendar API](https://developers.google.com/calendar/quickstart/python) with scope `https://www.googleapis.com/auth/calendar.readonly` in the OAuth consent screen. You will either need to make your OAuth app public or add your account to the list of test users. Configure the full `/oauth` URL of your App Engine app as an authorized redirect URI for the OAuth client. Write OAuth Client ID and Client Secret to the `client_id` and `client_secret` fields of the `google_calendar` document in the `oauth_client` Firestore collection (`/oauth_clients/google_calendar`).
+3. Obtain an [OAuth client ID](https://console.developers.google.com/apis/credentials) for the [Google Calendar API](https://developers.google.com/calendar/quickstart/python) with scope `https://www.googleapis.com/auth/calendar.readonly` in the OAuth consent screen. You will either need to make your OAuth app public or add your account to the list of test users. Configure the full `/oauth` URL of your App Engine app as an authorized redirect URI for the OAuth client. Write OAuth Client ID and Client Secret to the `client_id` and `client_secret` fields of the `google_calendar` document in the `oauth_clients` Firestore collection (`/oauth_clients/google_calendar`).
 
 To test and deploy the server:
 1. Install the [Google Cloud SDK](https://cloud.google.com/sdk/docs/), [create a project](https://cloud.google.com/resource-manager/docs/creating-managing-projects), and [authenticate with a service account](https://cloud.google.com/docs/authentication/getting-started).