docs(readme): update README.md

From GITHUB 393

COPYBARA_INTEGRATE_REVIEW=https://github.com/MegEngine/MegEngine/pull/393 from mlbo:patch-1 fe10cf5bc7

 Author:    iotboy <iotboy@163.com>

Closes #393

GitOrigin-RevId: 2dfc762e03

imperative/README.md

@@ -94,6 +94,6 @@ Tensor-likes have a dataflow semantic, thus immutable. `TensorWrapper` provide a
 
    * op always return a tuple
 
-      `result, = outputs`
+      `result = outputs`
 
    * input can be any tensor-like