docs: fix image demo

src/form/demo/FieldTypeArea.vue

@@ -1,10 +1,10 @@
 <template>
   <van-field
+    v-model="value"
     readonly
     clickable
     name="area"
     :label="t('picker')"
-    :model-value="value"
     :placeholder="t('placeholder')"
     @click="showArea = true"
   />

src/form/demo/FieldTypeCalendar.vue

@@ -1,10 +1,10 @@
 <template>
   <van-field
+    v-model="value"
     readonly
     clickable
     name="calendar"
     :label="t('calendar')"
-    :model-value="value"
     :placeholder="t('placeholder')"
     @click="showCalendar = true"
   />

src/form/demo/FieldTypeDatetimePicker.vue

@@ -1,10 +1,10 @@
 <template>
   <van-field
+    v-model="value"
     readonly
     clickable
     name="datetimePicker"
     :label="t('label')"
-    :model-value="value"
     :placeholder="t('placeholder')"
     @click="showPicker = true"
   />

src/form/demo/FieldTypePicker.vue

@@ -1,10 +1,10 @@
 <template>
   <van-field
+    v-model="value"
     readonly
     clickable
     name="picker"
     :label="t('picker')"
-    :model-value="value"
     :placeholder="t('placeholder')"
     @click="showPicker = true"
   />