fix(Banner): broken demo

Author: adamviktora

packages/react-core/src/demos/Banner.md

@@ -134,8 +134,8 @@ class BannerDemo extends React.Component {
               <PageSection>
                 <Gallery hasGutter>
                   {Array.from({ length: 30 }).map((_value, index) => (
-                    <GalleryItem key={i}>
-                      <Card key={i}>
+                    <GalleryItem key={index}>
+                      <Card key={index}>
                         <CardBody>This is a card</CardBody>
                       </Card>
                     </GalleryItem>