bgpd: Drop newline in JSON output for `show bgp afi safi json detail`

bgpd: Drop newline in JSON output for `show bgp afi safi json detail`

Before:

```
{
 "vrfId": 0,
 "vrfName": "default",
 "tableVersion": 2,
 "routerId": "1.1.1.1",
 "defaultLocPrf": 100,
 "localAS": 65001,
 "routes": { "192.168.1.0/24": {
"prefix": "192.168.1.0/24",
"version": "1",

"paths": [{"aspath":{"string":"Local","segments":[],"length":0},"origin":"IGP","metric":0,"weight":32768,"valid":true,"version":1,"sourced":true,"local":true,"bestpath":{"overall":true,"selectionReason":"First path received"},"lastUpdate":{"epoch":1713035588,"string":"Sat Apr 13 22:13:08 2024\n"},"nexthops":[{"ip":"0.0.0.0","hostname":"donatas.net","afi":"ipv4","metric":0,"accessible":true,"used":true}],"peer":{"peerId":"0.0.0.0","routerId":"1.1.1.1"}}]
} ,"192.168.11.0/24": {
"prefix": "192.168.11.0/24",
"version": "2",

"paths": [{"aspath":{"string":"Local","segments":[],"length":0},"origin":"IGP","metric":0,"weight":32768,"valid":true,"version":2,"sourced":true,"local":true,"bestpath":{"overall":true,"selectionReason":"First path received"},"lastUpdate":{"epoch":1713035588,"string":"Sat Apr 13 22:13:08 2024\n"},"nexthops":[{"ip":"0.0.0.0","hostname":"donatas.net","afi":"ipv4","metric":0,"accessible":true,"used":true}],"peer":{"peerId":"0.0.0.0","routerId":"1.1.1.1"}}]
}  }  }
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>