chore: fix typo

Author: ulivz

lib/app/dataMixin.js

@@ -50,7 +50,7 @@ export default {
       return this.$page.frontmatter.title || this.$localeConfig.title || this.$site.title || ''
     },
     $description () {
-      return this.$page.frontmatter.title || this.$localeConfig.description || this.$site.description || ''
+      return this.$page.frontmatter.description || this.$localeConfig.description || this.$site.description || ''
     },
     $lang () {
       return this.$page.frontmatter.lang || this.$localeConfig.lang || 'en-US'