Fix wrong help message for cable length setting (sonic-net#1978)

- What I did
Replace the wrong help message of "Set lossless PGs for the interface" with the right one "Set interface cable length"

- How I did it
Update the help message to the relevant one on main.py

- How to verify it
Run the following command: "config interface cable-length Ethernet0 -h"

- Previous command output (if the output of a command-line utility has changed)
Set lossless PGs for the interface

- New command output (if the output of a command-line utility has changed)
Set interface cable length

config/main.py

@@ -4110,7 +4110,7 @@ def remove_pg(ctx, interface_name, pg_map):
 @click.argument('length', metavar='<length>', required=True)
 @click.pass_context
 def cable_length(ctx, interface_name, length):
-    """Set lossless PGs for the interface"""
+    """Set interface cable length"""
     config_db = ctx.obj["config_db"]
 
     if not is_dynamic_buffer_enabled(config_db):