Update imageUrl method documentation

Adding the $word property to method documentation to match actual method implementation.

src/Faker/Generator.php

@@ -154,7 +154,7 @@
  * @property string $fileExtension
  * @method string file($sourceDirectory = '/tmp', $targetDirectory = '/tmp', $fullPath = true)
  *
- * @method string imageUrl($width = 640, $height = 480, $category = null, $randomize = true)
+ * @method string imageUrl($width = 640, $height = 480, $category = null, $randomize = true, $word = null)
  * @method string image($dir = null, $width = 640, $height = 480, $category = null, $fullPath = true)
  *
  * @property string $hexColor