docs: correct usage examples of useTemplateRef (#3031)

Author: DamianGlowala

src/guide/typescript/composition-api.md

@@ -376,7 +376,7 @@ With Vue 3.5 and `@vue/language-tools` 2.1 (powering both the IDE language servi
 In cases where auto-inference is not possible, you can still cast the template ref to an explicit type via the generic argument:
 
 ```ts
-const el = useTemplateRef<HTMLInputElement>(null)
+const el = useTemplateRef<HTMLInputElement>('el')
 ```
 
 <details>
@@ -438,7 +438,7 @@ In cases where the exact type of the component isn't available or isn't importan
 import { useTemplateRef } from 'vue'
 import type { ComponentPublicInstance } from 'vue'
 
-const child = useTemplateRef<ComponentPublicInstance | null>(null)
+const child = useTemplateRef<ComponentPublicInstance>('child')
 ```
 
 In cases where the component referenced is a [generic component](/guide/typescript/overview.html#generic-components), for instance `MyGenericModal`:
@@ -467,7 +467,7 @@ import { useTemplateRef } from 'vue'
 import MyGenericModal from './MyGenericModal.vue'
 import type { ComponentExposed } from 'vue-component-type-helpers'
 
-const modal = useTemplateRef<ComponentExposed<typeof MyGenericModal>>(null)
+const modal = useTemplateRef<ComponentExposed<typeof MyGenericModal>>('modal')
 
 const openModal = () => {
   modal.value?.open('newValue')