magento/adobe-stock-integration#1832: Updated fieldset name

Author: sivaschenko

app/code/Magento/MediaGalleryRenditions/etc/adminhtml/system.xml

@@ -9,7 +9,7 @@
     <system>
         <section id="system">
             <group id="media_gallery_renditions" translate="label" type="text" sortOrder="1010" showInDefault="1" showInWebsite="0" showInStore="0">
-                <label>Media Gallery Image Size</label>
+                <label>Media Gallery Image Optimization</label>
                 <comment>Resize images to improve performance and decrease the file size. When you use an image from Media Gallery on the storefront, the smaller image is generated and placed instead of the original.
                     Changing these settings will update all generated images.</comment>
                 <field id="width" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">