Improve logout example (#496)

The current example for logout is not show a complete example.

I have added a couple lines to handle the case where a logged out user tries to logout again.

Author: wt

actix-identity/src/lib.rs

@@ -74,8 +74,10 @@ async fn login(request: HttpRequest) -> impl Responder {
 }
 
 #[post("/logout")]
-async fn logout(user: Identity) -> impl Responder {
-    user.logout();
+async fn logout(user: Option<Identity>) -> impl Responder {
+    if let Some(user) = user {
+        user.logout();
+    }
     HttpResponse::Ok()
 }
 ```