Update the example for pfcwd start command (sonic-net#1984)

What I did
The example of pfcwd start command in start function comments is a little confused, after execution, it says invalid options.
Update it with a correct command.
The output of the current example is:
admin@vlab-01:~$ sudo pfcwd start --action drop ports all detection-time 400 --restoration-time 400
Failed to run command, invalid options:
ports
detection-time

How I did it
Update the example with a correct one in pfcwd start function comments which is also usage info for pfcwd start command.

How to verify it
Enter SONiC DUT, input the following command:
sudo pfcwd start --help
sudo pfcwd start --action drop all 400 --restoration-time 400

Signed-off-by: Zhaohui Sun <zhaohuisun@microsoft.com>

config/main.py

@@ -2061,7 +2061,7 @@ def start(action, restoration_time, ports, detection_time, verbose):
     Start PFC watchdog on port(s). To config all ports, use all as input.
 
     Example:
-        config pfcwd start --action drop ports all detection-time 400 --restoration-time 400
+        config pfcwd start --action drop all 400 --restoration-time 400
     """
     cmd = "pfcwd start"
 

doc/Command-Reference.md

@@ -6172,8 +6172,8 @@ This command starts PFC Watchdog
 
 - Usage:
   ```
-  config pfcwd start --action drop ports all detection-time 400 --restoration-time 400
-  config pfcwd start --action forward ports Ethernet0 Ethernet8 detection-time 400
+  config pfcwd start --action drop all 400 --restoration-time 400
+  config pfcwd start --action forward Ethernet0 Ethernet8 400
   ```
 
 **config pfcwd stop**

pfcwd/main.py

@@ -455,7 +455,7 @@ def start(db, action, restoration_time, ports, detection_time):
 
         Example:
 
-        sudo pfcwd start --action drop ports all detection-time 400 --restoration-time 400
+        sudo pfcwd start --action drop all 400 --restoration-time 400
 
         """
         PfcwdCli(db).start(

tests/pfcwd_test.py

@@ -118,7 +118,7 @@ def test_pfcwd_start_ports_valid(self, mock_os):
 
     @patch('pfcwd.main.os')
     def test_pfcwd_start_actions(self, mock_os):
-        # pfcwd start --action fwd --restoration-time 200 Ethernet0 200
+        # pfcwd start --action forward --restoration-time 200 Ethernet0 200
         import pfcwd.main as pfcwd
         runner = CliRunner()
         db = Db()