docs: replace WEBP with WebP

Author: kenjis

user_guide_src/source/libraries/images.rst

@@ -86,7 +86,7 @@ Image Quality
 
 ``save()`` can take an additional parameter ``$quality`` to alter the resulting image
 quality. Values range from 0 to 100 with 90 being the framework default. This parameter
-only applies to JPEG and WEBP images, will be ignored otherwise:
+only applies to JPEG and WebP images, will be ignored otherwise:
 
 .. note:: The parameter ``$quality`` for WebP can be used since v4.4.0.