Change branded wallpaper demo resource file name

Wallpaper resource name should follow NTPSponsoredImagesData naming
rule. NTPSponsoredImagesData renerates path like wallpaper-N.jpg.
So, resource name also should be XXX/wallpaper-N.jpg.

fix brave/brave-browser#7930

browser/ui/webui/brave_webui_source.cc

@@ -79,11 +79,11 @@ void CustomizeWebUIHTMLSource(const std::string &name,
         { "ntp-14.webp", IDR_BRAVE_NEW_TAB_BACKGROUND14 },
         { "ntp-15.webp", IDR_BRAVE_NEW_TAB_BACKGROUND15 },
         { "ntp-16.webp", IDR_BRAVE_NEW_TAB_BACKGROUND16 },
-        { "ntp-dummy-brandedwallpaper/wallpaper1.jpg",
+        { "ntp-dummy-brandedwallpaper/wallpaper-0.jpg",
             IDR_BRAVE_NEW_TAB_DUMMY_BRANDED_WALLPAPER_BG_1 },
-        { "ntp-dummy-brandedwallpaper/wallpaper2.jpg",
+        { "ntp-dummy-brandedwallpaper/wallpaper-1.jpg",
             IDR_BRAVE_NEW_TAB_DUMMY_BRANDED_WALLPAPER_BG_2 },
-        { "ntp-dummy-brandedwallpaper/wallpaper3.jpg",
+        { "ntp-dummy-brandedwallpaper/wallpaper-2.jpg",
             IDR_BRAVE_NEW_TAB_DUMMY_BRANDED_WALLPAPER_BG_3 },
         { "ntp-dummy-brandedwallpaper/logo.png",
             IDR_BRAVE_NEW_TAB_DUMMY_BRANDED_WALLPAPER_LOGO },