README: update type of StorageProvider interface

Author: xr0master

README.md

@@ -153,9 +153,9 @@ The custom provider must match the interface.
 
 ```ts
 interface StorageProvider {
-  get: (key: string) => string | null | undefined;
-  set: (key: string, value: string) => void;
-  remove: (key: string) => void;
+  get: (key: string) => Promise<string | null | undefined>;
+  set: (key: string, value: string) => Promise<void>;
+  remove: (key: string) => Promise<void>;
 }
 ```