fix: setting offset breaks "centered" plugin #394 (#618)

* change padding to margin to avoid spacing issues with box sizing: border-box

Author: RobertHebel

react-carousel/src/components/CarouselSlide.js

@@ -72,8 +72,8 @@ const CarouselSlide = ({
         ...(itemClassNames || []),
       )}
       style={{
-        paddingRight: `${offset / 2}px`,
-        paddingLeft: `${offset / 2}px`,
+        marginRight: `${offset / 2}px`,
+        marginLeft: `${offset / 2}px`,
         width: `${width}px`,
         maxWidth: `${width}px`,
         minWidth: `${width}px`,