docs($core): fix typos (#2381)

* perf($core): fix typos

* docs ($core): update casing
Co-authored-by: NBen Hong <ben@bencodezen.io>

packages/@vuepress/core/lib/client/root-mixins/updateMeta.js

@@ -95,7 +95,7 @@ function metaIdentifier (tag) {
  * Render meta tags
  *
  * @param {Array} meta
- * @returns {Array<string>}
+ * @returns {String}
  */
 
 function renderPageMeta (meta) {