Add tc command description to usage.txt. (ipspace#2670)

Fixes ipspace#2670

Author: DanPartelly

netsim/cli/usage.txt

@@ -29,6 +29,9 @@ capture     Start packet capture on the specified node/interface
 
 exec        Executes a command on one or more network devices
 
+tc          Disable, enable, display, or modify link impairment parameters on 
+            libvirt virtual machines or containerlab-created Docker containers.
+
 [bold]Reports and graphs[/bold]
 ==================
 status      Display the state of lab instances running on the current server