docs: Fix trace tcp header.
Fixes: ("docs: Add tcptracer guide.")
@@ -21,7 +21,7 @@ You can now use the gadget, but output will be empty:
```bash
$ kubectl gadget trace tcp
-NODE NAMESPACE POD CONTAINER KPID KCOMM PAGES TPID TCOMM
+NODE NAMESPACE POD CONTAINER T PID COMM IP SADDR DADDR SPORT DPORT
```
Indeed, it is waiting for TCP connection to be established in the `default` namespace (you can use `-A` to monitor all namespaces and then be sure to not miss any event).
