docs(Form): use round popup

src/form/demo/FieldTypeArea.vue

@@ -10,6 +10,7 @@
   >
     <van-popup
       v-model="showArea"
+      round
       slot="extra"
       position="bottom"
       get-container="body"

src/form/demo/FieldTypeCalendar.vue

@@ -10,6 +10,7 @@
   >
     <van-calendar
       v-model="showCalendar"
+      round
       slot="extra"
       get-container="body"
       @confirm="onConfirm"

src/form/demo/FieldTypeDatetimePicker.vue

@@ -10,6 +10,7 @@
   >
     <van-popup
       v-model="showPicker"
+      round
       slot="extra"
       position="bottom"
       get-container="body"

src/form/demo/FieldTypePicker.vue

@@ -10,6 +10,7 @@
   >
     <van-popup
       v-model="showPicker"
+      round
       slot="extra"
       position="bottom"
       get-container="body"