docs: update docs

Author: hunterliu1003

docs/content/3.api/2.composables/1.use-modal.md

@@ -94,7 +94,7 @@ You have to manually calling `modalInstance.destroy()`{lang=ts} whenever you no
   import { VueFinalModal, useModal } from 'vue-final-modal'
   import { vfm } from '@/plugins/vue-final-modal'
 
-  function openConfirmModal() {
+  export function openConfirmModal() {
     const modalInstance = useModal({
       context: vfm,
       component: markRaw(MyConfirmModal),
@@ -130,7 +130,7 @@ You have to manually calling `modalInstance.destroy()`{lang=ts} whenever you no
   import { VueFinalModal, useModal } from 'vue-final-modal'
   import { vfm } from '@/plugins/vue-final-modal'
 
-  function openConfirmModal() {
+  export function openConfirmModal() {
     const modalInstance = useModal({
       context: vfm,
       component: markRaw(MyConfirmModal),