[HttpFoundation] Fixed PHP doc of ParameterBag::getBoolean

Since the method should return a bool, then the default value should be
a bool too

src/Symfony/Component/HttpFoundation/ParameterBag.php

@@ -231,7 +231,7 @@ class ParameterBag implements \IteratorAggregate, \Countable
      * Returns the parameter value converted to boolean.
      *
      * @param string $key     The parameter key
-     * @param mixed  $default The default value if the parameter key does not exist
+     * @param bool   $default The default value if the parameter key does not exist
      * @param bool   $deep    If true, a path like foo[bar] will find deeper items
      *
      * @return bool The filtered value