Fix call for spanning-tree commands in dump script (sonic-net#3723)

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

scripts/generate_dump

@@ -2209,10 +2209,10 @@ main() {
     wait
 
  	save_cmd "stpctl all" "stp.log"
-    save_cmd "show spanning_tree" "stp.show"
-    save_cmd "show spanning_tree statistics" "stp.stats"
-    save_cmd "show spanning_tree bpdu_guard" "stp.bg"
-    save_cmd "show spanning_tree root_guard" "stp.rg"
+    save_cmd "show spanning-tree" "stp.show"
+    save_cmd "show spanning-tree statistics" "stp.stats"
+    save_cmd "show spanning-tree bpdu_guard" "stp.bg"
+    save_cmd "show spanning-tree root_guard" "stp.rg"
 
     save_cmd "ps aux" "ps.aux" &
     save_cmd "top -b -n 1" "top" &