Button: optimize scss(#11900)

packages/theme-chalk/src/button.scss

@@ -21,7 +21,7 @@
   transition: .1s;
   font-weight: $--button-font-weight;
   @include utils-user-select(none);
-  & + .el-button {
+  & + & {
     margin-left: 10px;
   }