Update OnSerializingAttribute.xml (#7315)

Summary missing the word "called", similar to the other serialization attributes.

Author: DavinMickelson

xml/System.Runtime.Serialization/OnSerializingAttribute.xml

@@ -54,7 +54,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>When applied to a method, specifies that the method is during serialization of an object in an object graph. The order of serialization relative to other objects in the graph is non-deterministic.</summary>
+    <summary>When applied to a method, specifies that the method is called during serialization of an object in an object graph. The order of serialization relative to other objects in the graph is non-deterministic.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[