fix authProvider doc

blog/2020-03-custom-providers.md

@@ -176,9 +176,7 @@ The constructor of our class will look something like this:
 
 ```js
 class MyCustomProvider {
-	constructor(options) {
-		this.authProvider = 'myunsplash'; // the name of our provider (lowercased)
-	}
+	static authProvider = 'google'; // the name of the OAuth provider to use (lowercased)
 	// ...
 }
 ```
@@ -305,9 +303,7 @@ const request = require('request');
 const BASE_URL = 'https://api.unsplash.com';
 
 class MyCustomProvider {
-	constructor(options) {
-		this.authProvider = 'myunsplash';
-	}
+	static authProvider = 'unsplash'; // the name of the OAuth provider to use (lowercased)
 
 	list({ token, directory }, done) {
 		const path = directory ? `/${directory}/photos` : '';

docs/companion.md

@@ -828,7 +828,9 @@ with an `Error`):
 The class must also have:
 
 - A unique `static authProvider` string property - a lowercased value which
-  typically indicates the name of the provider (e.g “dropbox”).
+  indicates name of the [`grant`](https://github.com/simov/grant) OAuth2
+  provider to use (e.g `google` for Google). If your provider doesn’t use
+  OAuth2, you can omit this property.
 - A `static` property `static version = 2`, which is the current version of the
   Companion Provider API.