docs(ui-byline): use the illu-sm icon size in the README example

The spacing/icon migration set the Byline hero icon to size="md"; the
correct illustrative size for this decorative icon is "illu-sm".

INSTUI-5118

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: git-nandor

packages/ui-byline/src/Byline/v2/README.md

@@ -54,6 +54,6 @@ type: example
       </Text>
     </View>
   }>
-  <HeartInstUIIcon size="md" color="successColor" />
+  <HeartInstUIIcon size="illu-sm" color="successColor" />
 </Byline>
 ```