bugfix: incorrect command for portchannel creation (sonic-net#2134)

#### What I did
Fix command for portchannel creation

#### How I did it
Change the command line guide:
Before:
config portchannel (add | del) [min-links ] [fallback (true | false)]
After:
config portchannel (add | del) [--min-links ] [--fallback (true | false)]

doc/Command-Reference.md

@@ -6694,7 +6694,7 @@ A port channel can be deleted only if it does not have any members or the member
 
 - Usage:
   ```
-  config portchannel (add | del) <portchannel_name> [min-links <num_min_links>] [fallback (true | false)]
+  config portchannel (add | del) <portchannel_name> [--min-links <num_min_links>] [--fallback (true | false)]
   ```
 
 - Example (Create the portchannel with name "PortChannel0011"):