fix: slider style error vueComponent#2097

close vueComponent#2097

Author: tangjinzhou

components/slider/style/index.less

@@ -160,7 +160,7 @@
     }
 
     .@{slider-prefix-cls}-handle {
-      margin-bottom: -7px;
+      margin-top: -6px;
       margin-left: -5px;
     }