Fix method arguments for session::flash() in the Facade. (#37625)

Author: BenCavens

src/Illuminate/Support/Facades/Session.php

@@ -13,7 +13,7 @@
  * @method static bool save()
  * @method static bool start()
  * @method static mixed get(string $key, $default = null)
- * @method static mixed flash(string $class, string $message)
+ * @method static mixed flash(string $key, $value = true)
  * @method static mixed pull(string $key, $default = null)
  * @method static mixed remove(string $key)
  * @method static string getId()