Update JsonPolymorphicAttribute.xml (#10416)

Author: eiriktsarpalis

xml/System.Text.Json.Serialization/JsonPolymorphicAttribute.xml

@@ -92,7 +92,7 @@
         <ReturnType>System.String</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a custom type discriminator property name for the polymorhic type.
+        <summary>Gets or sets a custom type discriminator property name for the polymorphic type.
             Uses the default '$type' property name if left unset.</summary>
         <value>To be added.</value>
         <remarks>To be added.</remarks>