Fixed typo.

Cake/Network/Request.php

@@ -995,7 +995,7 @@ public function cookie($key) {
  * Fallback to using env() if key not set in $environment property.
  *
  * @param string $key The key you want to read/write from/to.
- * @param string $value Value to set. Defaut null.
+ * @param string $value Value to set. Default null.
  * @return null|string|Cake\Network\Request Request instance if used as setter,
  *   if used as getter either the environment value, or null if the value doesn't exist.
  */