fix: use 'let' in an unwatch example as 'const' causes an error (#584)

Author: skirtles-code

src/api/instance-methods.md

@@ -158,7 +158,9 @@
   If you still want to call an unwatch function inside the callback, you should check its availability first:
 
   ```js
-  const unwatch = vm.$watch(
+  let unwatch = null
+  
+  unwatch = vm.$watch(
     'value',
     function() {
       doSomething()