Correct giveConfig param doc (#41314)

Author: Neol3108

src/Illuminate/Container/ContextualBindingBuilder.php

@@ -86,7 +86,7 @@ public function giveTagged($tag)
      * Specify the configuration item to bind as a primitive.
      *
      * @param  string  $key
-     * @param  ?string  $default
+     * @param  mixed  $default
      * @return void
      */
     public function giveConfig($key, $default = null)

src/Illuminate/Contracts/Container/ContextualBindingBuilder.php

@@ -32,7 +32,7 @@ public function giveTagged($tag);
      * Specify the configuration item to bind as a primitive.
      *
      * @param  string  $key
-     * @param  ?string  $default
+     * @param  mixed  $default
      * @return void
      */
     public function giveConfig($key, $default = null);