Update usage (#43)

Co-authored-by: Renée <renee@kooi.me>

README.md

@@ -30,22 +30,24 @@ var Keyv = require('keyv')
 var KeyvFirestore = require('keyv-firestore')
 
 const keyv = new Keyv({
-  store: KeyvFirestore,
-  // REQUIRED: the project id to use
-  projectId: 'my-firebase-project',
-  // REQUIRED: the root collection to store things in
-  collection: 'firestore-db-collection'
+  store: new KeyvFirestore({
+    // REQUIRED: the project id to use
+    projectId: 'my-firebase-project',
+    // REQUIRED: the root collection to store things in
+    collection: 'firestore-db-collection'
+  })
 })
 ```
 
 You can set the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to configure credentials, or pass in a JavaScript object in the `credentials` option.
 
 ```js
 const keyv = new Keyv({
-  store: KeyvFirestore,
-  projectId: 'my-firebase-project',
-  collection: 'firestore-db-collection',
-  credentials: require('./path/to/google-credentials.json')
+  store: new KeyvFirestore({
+    projectId: 'my-firebase-project',
+    collection: 'firestore-db-collection',
+    credentials: require('./path/to/google-credentials.json')
+  })
 })
 ```