docs(Field): optimize docs (#6527)

src/field/README.md

@@ -198,8 +198,8 @@ Use `input-align` prop to align the input value
 ```html
 <van-field
   v-model="value"
-  :label="Text"
+  label="Text"
-  :placeholder="Input Align Right"
+  placeholder="Input Align Right"
   input-align="right"
 />
 ```

src/field/README.zh-CN.md

@@ -223,8 +223,8 @@ export default {
 ```html
 <van-field
   v-model="value"
-  :label="文本"
+  label="文本"
-  :placeholder="输入框内容右对齐"
+  placeholder="输入框内容右对齐"
   input-align="right"
 />
 ```