Update assertExists() magic method signature of Storage facade (#44668)

Author: rjorel

src/Illuminate/Support/Facades/Storage.php

@@ -5,7 +5,7 @@
 use Illuminate\Filesystem\Filesystem;
 
 /**
- * @method static \Illuminate\Contracts\Filesystem\Filesystem assertExists(string|array $path)
+ * @method static \Illuminate\Contracts\Filesystem\Filesystem assertExists(string|array $path, string|null $content = null)
  * @method static \Illuminate\Contracts\Filesystem\Filesystem assertDirectoryEmpty(string $path)
  * @method static \Illuminate\Contracts\Filesystem\Filesystem assertMissing(string|array $path)
  * @method static \Illuminate\Contracts\Filesystem\Filesystem cloud()