docs: cli-command-syntax.md: Missing arg to show VRF RIB

This patch fixes the example on the doc for showing the VRF RIB where
the "rib" keyword is missing to execute the command.

Also, the "-a <adddress family>" is an option and this patch puts it
into brackets.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>

docs/sources/cli-command-syntax.md

@@ -428,11 +428,11 @@ If you want to remove one element(extended community) of ExtCommunitySet, to spe
 #### Syntax
 ```shell
 # add routes to vrf
-% gobgp vrf <vrf name> rib add <prefix> -a <address family>
+% gobgp vrf <vrf name> rib add <prefix> [-a <address family>]
 # del routes from vrf
-% gobgp vrf <vrf name> rib del <prefix> -a <address family>
+% gobgp vrf <vrf name> rib del <prefix> [-a <address family>]
 # show routes in vrf
-% gobgp vrf <vrf name>
+% gobgp vrf <vrf name> rib [-a <address family>]
 ```
 
 #### Example