tftp: add verification help/example

Added verification section so that it will in testing the feature

(cherry picked from commit b13fd424a155ab09904cdbab45051a77adf1e6a1)

docs/configuration/service/tftp-server.rst

@@ -52,3 +52,21 @@ The resulting configuration will look like:
       listen-address 2001:db8::1
       listen-address 192.0.2.1
    }
+
+Verification
+------------
+
+Client:
+
+.. code-block:: none
+
+  vyos@RTR2:~$ tftp -p -l /config/config.boot -r backup 192.0.2.1
+  backup1             100% |******************************|   723  0:00:00 ETA
+
+Server:
+
+.. code-block:: none
+
+  vyos@RTR1# ls -ltr /config/tftpboot/
+  total 1
+  -rw-rw-rw- 1 tftp tftp  1995 May 19 16:02 backup