Tweak README to reflect conflict? flag changes (#1092)

Since `Oban.insert` changed its behaviour regarding the conflict?
when failing to acquire the respective advisory lock the README should
reflect that.

README.md

@@ -517,8 +517,15 @@ otherwise it is `false`.
 You can use the `:conflict?` field to customize responses after insert:
 
 ```elixir
-with {:ok, %Job{conflict?: true}} <- Oban.insert(changeset) do
-  {:error, :job_already_exists}
+case Oban.insert(changeset) do
+  {:ok, %Job{id: nil, conflict?: true}} ->
+    {:error, :failed_to_acquire_lock}
+
+  {:ok, %Job{conflict?: true}} ->
+    {:error, :job_already_exists}
+
+  result ->
+    result
 end
 ```