use the right size icons

page-to-extension-messaging/icons/LICENSE

@@ -1,2 +1,2 @@
 
-The "message-32.png" and "message-64.png" icons are taken from the miu iconset created by Linh Pham Thi Dieu, and are used under the terms of its license: http://linhpham.me/miu/.
+The "message-48.png" icon is taken from the miu iconset created by Linh Pham Thi Dieu, and is used under the terms of its license: http://linhpham.me/miu/.

page-to-extension-messaging/manifest.json

@@ -6,8 +6,7 @@
   "version": "1.0",
   "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/page-to-extension-messaging",
   "icons": {
-    "32": "icons/message-32.png",
-    "64": "icons/message-64.png"
+    "48": "icons/message-48.png"
   },
 
   "applications": {