ImageManipulation.php API documentation fix

The API documentation for ImageManipulation has a minor problem. The ThumbnailIcon function description is missing the `<img />` tag text in the description:
https://api.silverstripe.org/4/SilverStripe/Assets/ImageManipulation.html#method_ThumbnailIcon

This is because the `<img />` tag is unescaped in the function documentation. 

This fix changes the tag to be escaped.

src/ImageManipulation.php

@@ -660,7 +660,7 @@ public function Thumbnail($width, $height)
     /**
      * Thubnail generation for all file types.
      *
-     * Resizes images, but returns an icon <img /> tag if this is not a resizable image
+     * Resizes images, but returns an icon `<img />` tag if this is not a resizable image
      *
      * @param int $width
      * @param int $height