Add $format to image DocBlock

[vintagesucks]

What is the reason for this PR?

• fixes a DocBlock

Similar to #630, the image DocBlock is missing the $format parameter:

Faker/src/Faker/Provider/Image.php

Lines 108 to 118 in 5c1e500

	public static function image(
	$dir = null,
	$width = 640,
	$height = 480,
	$category = null,
	$fullPath = true,
	$randomize = true,
	$word = null,
	$gray = false,
	$format = 'png'
	) {

Faker/src/Faker/Generator.php

Line 282 in 5c1e500

* @method string image($dir = null, $width = 640, $height = 480, $category = null, $fullPath = true, $randomize = true, $word = null, $gray = false)

Author's checklist

• Follow the Contribution Guide
• New features and changes are documented

Summary of changes

Adds $format to image DocBlock

Review checklist

• All checks have passed
• Changes are approved by maintainer