build(deps): bump @vuepress/plugin-back-to-top from 1.8.0 to 1.8.1

Bumps [@vuepress/plugin-back-to-top](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/plugin-back-to-top) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/v1.8.1/packages/@vuepress/plugin-back-to-top)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -17,7 +17,7 @@
     }
   },
   "dependencies": {
-    "@vuepress/plugin-back-to-top": "^1.8.0",
+    "@vuepress/plugin-back-to-top": "^1.8.1",
     "@vuepress/plugin-google-analytics": "^1.8.0",
     "vuepress": "^1.8.0"
   },