Update dependency vuepress to v2.0.0-beta.50-pre.1

Author: renovate[bot]

package.json

@@ -11,7 +11,7 @@
   "devDependencies": {
     "@vuepress/client": "2.0.0-beta.51",
     "vue": "3.2.40",
-    "vuepress": "2.0.0-beta.51",
+    "vuepress": "2.0.0-beta.50-pre.1",
     "vuepress-theme-hope": "2.0.0-beta.109"
   }
 }