Fix link to docs on custom inputs (#1660)

The old link redirected me to https://vuejs.org/v2/guide/components-custom-events.html#Binding-Native-Events-to-Components which was unexpected. I think the correct link is https://vuejs.org/v2/guide/components.html#Using-v-model-on-Components as per this change.

Author: alastairs

src/v2/guide/forms.md

@@ -382,4 +382,4 @@ If you want user input to be trimmed automatically, you can add the `trim` modif
 
 > If you're not yet familiar with Vue's components, you can skip this for now.
 
-HTML's built-in input types won't always meet your needs. Fortunately, Vue components allow you to build reusable inputs with completely customized behavior. These inputs even work with `v-model`! To learn more, read about [custom inputs](components.html#Form-Input-Components-using-Custom-Events) in the Components guide.
+HTML's built-in input types won't always meet your needs. Fortunately, Vue components allow you to build reusable inputs with completely customized behavior. These inputs even work with `v-model`! To learn more, read about [custom inputs](components.html#Using-v-model-on-Components) in the Components guide.