docs: fix typo (#1510)

Author: brown121407

packages/docs/docs/plugin/README.md

@@ -5,6 +5,6 @@ Plugins usually add global-level functionality to VuePress. There is no strictly
 1. Extend the page's metadata generated at compile time. e.g. [@vuepress/plugin-last-updated](./official/plugin-last-updated.md);
 2. Generate extra files before or after compilation. e.g. [@vuepress/plugin-pwa](./official/plugin-pwa.md);
 3. Inject global UI. e.g. [@vuepress/plugin-back-to-top](./official/plugin-back-to-top.md);
-4. Exntend command of CLI，如 [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export)。
+4. Extend the CLI with custom commands. e.g. [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export).
 
 ![Architecture of VuePress](/architecture.png)