Docs: fix Form and Input demo style (#15273)

examples/demo-styles/form.scss

@@ -1,4 +1,4 @@
-.demo-form.demo-zh-CN {
+.demo-form {
   .el-select .el-input {
     width: 380px;
   }

examples/demo-styles/input.scss

@@ -1,4 +1,4 @@
-.demo-input.demo-zh-CN {
+.demo-input {
   .el-select .el-input {
     width: 130px;
   }