[docs][tune] Fix Tune tutorial (ray-project#34660)

One line fix for bug introduced in ray-project#34435

Signed-off-by: Kai Fricke <kai@anyscale.com>
Signed-off-by: elliottower <elliot@elliottower.com>

python/ray/tune/tests/tutorial.py

@@ -106,7 +106,7 @@ def train_mnist(config):
         acc = test(model, test_loader)
 
         # Send the current training result back to Tune
-        session.report(mean_accuracy=acc)
+        session.report({"mean_accuracy": acc})
 
         if i % 5 == 0:
             # This saves the model to the trial directory