Fix gitbook sidebar plugin

Author: amakarudze

book.json

@@ -22,7 +22,7 @@
         "[email protected]",
         "[email protected]",
         "language-picker",
-        "sidebar-ads",
+        "sidebar-ad",
         "codeblock-label",
         "sectionx-ex",
         "collapsible-menu"
@@ -40,7 +40,7 @@
         "language-picker": {
             "grid-columns": 3
         },
-        "sidebar-ads": {
+        "sidebar-ad": {
             "imageUrl": "https://djangogirls.org/static/img/global/donate/lagos.jpg",
             "url": "https://www.patreon.com/djangogirls",
             "description": "💖 Support our work and donate to our project! ✨",