📝 docs: Fix typos (#29) by @ismailarilik

Author: ismailarilik

docs/rules/no-unused-keys.md

@@ -88,4 +88,4 @@ i18n.t('hi')
 ```
 
 - `src`: specify the source codes directory to be able to lint. If you don't set any options, it set to `process.cwd()` as default.
-- `extenstions`: an array to allow specified lintable target file extention. If you don't set any options, it set to `.js` and` .vue` as default.
+- `extensions`: an array to allow specified lintable target file extension. If you don't set any options, it set to `.js` and` .vue` as default.