bgpd: fix missing braces when dumping json vpn advertised-routes

bgpd: fix missing braces when dumping json vpn advertised-routes

The json output of advertised-routes is incorrect, as there is a missing
brace with route-distinguisher:

observed with the bgp_vpnv4_noretain test:
> "bgpTableVersion":0,"bgpLocalRouterId":"192.0.2.1","defaultLocPrf":100,"localAS":65500,
> "advertisedRoutes": "192.0.2.1:1":{"rd":"192.0.2.1:1","10.101.0.0/24":{"prefix":"10.101.0.0/24",

expected:
> "bgpTableVersion":0,"bgpLocalRouterId":"192.0.2.1","defaultLocPrf":100,"localAS":65500,
> "advertisedRoutes": { "192.0.2.1:1":{"rd":"192.0.2.1:1","10.101.0.0/24":{"prefix":"10.101.0.0/24",
>                     ^
>                     missing brace

Fix this by adding the missing braces.

Fixes: 4838bac033a7 ("bgpd: neighbors received-routes/advertised-routes stringify changes")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>