Fix crop_aspect_ratio anchor

Author: DarthSim

docs/usage/processing.mdx

@@ -296,7 +296,7 @@ Defines an area of the image to be processed (crop before resize).
   * When `width` or `height` is set to `0`, imgproxy will use the full width/height of the source image.
 * `gravity` _(optional)_ accepts the same values as the [gravity](#gravity) option. When `gravity` is not set, imgproxy will use the value of the [gravity](#gravity) option.
 
-### Crop aspect ratio ((pro))
+### Crop aspect ratio ((pro)) {#crop-aspect-ratio}
 
 ```imgproxy_url_option
 crop_aspect_ratio:%aspect_ratio:%enlarge