style: change task manage module top filter placeholder color

src/pages/TaskManage/index.less

@@ -29,6 +29,10 @@
   font-size: 20px;
 }
 
+.antdSelect .ant-select-selection-placeholder{
+  color: rgba(0,0,0,0.75);
+}
+
 .ant-select-dropdown{
   width: unset;
 }