docs: fix back-to-top plugin package.json homepage URL 404 (#2646)

Author: Igloczek

packages/@vuepress/plugin-back-to-top/package.json

@@ -8,7 +8,7 @@
     "vue",
     "vuepress"
   ],
-  "homepage": "https://github.com/vuejs/vuepress/blob/master/packages/@vuepress/back-to-top#readme",
+  "homepage": "https://github.com/vuejs/vuepress/tree/master/packages/%40vuepress/plugin-back-to-top#readme",
   "bugs": {
     "url": "https://github.com/vuejs/vuepress/issues"
   },