Fix warning in waitall doc

python/mxnet/ndarray/ndarray.py

@@ -157,11 +157,13 @@ def waitall():
     """Wait for all async operations to finish in MXNet.
 
     This function is used for benchmarking only.
+
     .. warning::
-    If your code has exceptions, `waitall` can cause silent failures.
-    For this reason you should avoid `waitall` in your code.
-    Use it only if you are confident that your code is error free.
-    Then make sure you call `wait_to_read` on all outputs after `waitall`.
+
+       If your code has exceptions, `waitall` can cause silent failures.
+       For this reason you should avoid `waitall` in your code.
+       Use it only if you are confident that your code is error free.
+       Then make sure you call `wait_to_read` on all outputs after `waitall`.
     """
     check_call(_LIB.MXNDArrayWaitAll())