doc(custom-drivers): update functions signatures

Author: Exilz

docs/custom-drivers.md

@@ -4,10 +4,10 @@ This wrapper has been written with the goal of **being storage-agnostic**. This
 
 Your custom driver must implement these 3 methods that are promises.
 
-* `getItem(key: string, callback?: (error?: Error, result?: string) => void)`
-* `setItem(key: string, value: string, callback?: (error?: Error) => void);`
-* `removeItem(key: string, callback?: (error?: Error) => void);`
-* `multiRemove(keys: string[], callback?: (errors?: Error[]) => void);`
+* `getItem(key: string): Promise<any>;`
+* `setItem(key: string, value: string): Promise<void>;`
+* `removeItem(key: string): Promise<void>;`
+* `multiRemove(keys: string[]): Promise<void>;`
 
 ## SQLite Driver