adjust a note format (#7812)

Author: Shen Yuanyu

docs/concepts/overview/working-with-objects/labels.md

@@ -58,6 +58,7 @@ An empty label selector (that is, one with zero requirements) selects every obje
 A null label selector (which is only possible for optional selector fields) selects no objects.
 
 **Note**: the label selectors of two controllers must not overlap within a namespace, otherwise they will fight with each other.
+{: .note}
 
 ### _Equality-based_ requirement