Fix call for spanning-tree commands in dump script

[DavidZagury]

What I did

Fix the calls for spanning-tree commands in dump script.
During call to generate techsupport, we can see that the spanning tree commands fail:

.
Error: No such command "spanning_tree".
timeout --foreground 5m bash -c "dummy_cleanup_method ()
.

How I did it

Change from show spanning_tree to the actual command show spanning-tree

How to verify it

Call show techsupport

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)