Fixed malformed XML

Author: Ron Petrusha

xml/System.Text.Json.Serialization/JsonExtensionDataAttribute.xml

@@ -51,7 +51,7 @@ If there is more than one extension property on a type, or it the property is no
         <AssemblyVersion>4.0.0.0</AssemblyVersion>
       </AssemblyInfo>
       <Parameters />
-      <Docsd
+      <Docs>
         <summary>Instantiates a new instance of the <see cref="T:System.Text.Json.Serialization.JSonExtensionDataAttribute" /> class.</summary>
         <remarks>To be added.</remarks>
       </Docs>