Update GenericIdentity.xml (#7828)

Delete unneeded verb 'was' in "if the user was has been authenticated..."

Author: Nicholas Piazza

xml/System.Security.Principal/GenericIdentity.xml

@@ -424,7 +424,7 @@
       <Docs>
         <summary>Gets a value indicating whether the user has been authenticated.</summary>
         <value>
-          <see langword="true" /> if the user was has been authenticated; otherwise, <see langword="false" />.</value>
+          <see langword="true" /> if the user has been authenticated; otherwise, <see langword="false" />.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>