fix typo gaurds -> guards (#676)

Author: ktsn

docs/en/api/router-instance.md

@@ -25,7 +25,7 @@
 - **router.beforeEach(guard)**
 - **router.afterEach(hook)**
 
-  Add global navigation guards. See [Navigation Guards](../advanced/navigation-gaurds.md).
+  Add global navigation guards. See [Navigation Guards](../advanced/navigation-guards.md).
 
 
 - **router.push(location)**