chore(deps-dev): bump @vuepress/plugin-back-to-top from 1.9.8 to 1.9.9

Bumps [@vuepress/plugin-back-to-top](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/plugin-back-to-top) from 1.9.8 to 1.9.9.
- [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/v1.9.9/packages/@vuepress/plugin-back-to-top)

---
updated-dependencies:
- dependency-name: "@vuepress/plugin-back-to-top"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

package-lock.json

@@ -7,7 +7,7 @@
   "devDependencies": {
     "@centerforopenscience/markdown-it-video": "^1.0.0",
     "@vuepress/plugin-active-header-links": "^1.9.8",
-    "@vuepress/plugin-back-to-top": "^1.9.8",
+    "@vuepress/plugin-back-to-top": "^1.9.9",
     "@vuepress/plugin-google-analytics": "^1.9.8",
     "@vuepress/plugin-html-redirect": "^0.1.4",
     "@vuepress/plugin-last-updated": "^1.9.8",