Fixed documentation typo (#1862)

Corrected a typo on the XML documentation of the UserId property that made it read "... sets the of the ..."

src/Stores/IdentityUserLogin.cs

@@ -27,8 +27,8 @@ namespace Microsoft.AspNetCore.Identity
         public virtual string ProviderDisplayName { get; set; }
 
         /// <summary>
-        /// Gets or sets the of the primary key of the user associated with this login.
+        /// Gets or sets the primary key of the user associated with this login.
         /// </summary>
         public virtual TKey UserId { get; set; }
     }
-}
+}