docs(Form): fix form basic usage document errors (#13387)

packages/vant/src/form/README.md

@@ -27,7 +27,7 @@ app.use(CellGroup);
   <van-cell-group inset>
     <van-field
       v-model="username"
-      name="Username"
+      name="username"
       label="Username"
       placeholder="Username"
       :rules="[{ required: true, message: 'Username is required' }]"
@@ -35,7 +35,7 @@ app.use(CellGroup);
     <van-field
       v-model="password"
       type="password"
-      name="Password"
+      name="password"
       label="Password"
       placeholder="Password"
       :rules="[{ required: true, message: 'Password is required' }]"

packages/vant/src/form/README.zh-CN.md

@@ -29,7 +29,7 @@ app.use(CellGroup);
   <van-cell-group inset>
     <van-field
       v-model="username"
-      name="用户名"
+      name="username"
       label="用户名"
       placeholder="用户名"
       :rules="[{ required: true, message: '请填写用户名' }]"
@@ -37,7 +37,7 @@ app.use(CellGroup);
     <van-field
       v-model="password"
       type="password"
-      name="密码"
+      name="password"
       label="密码"
       placeholder="密码"
       :rules="[{ required: true, message: '请填写密码' }]"