bgpd: fix help for ipv6 under distance src command

bgpd: fix help for ipv6 under distance src command

We were missing the help docstring for IPV6 under the
distance src command in for bgp.

```
[root@alfred frr-2]# /usr/lib/frr/bgpd --log stdout --log-level debug
2020/10/30 16:02:26 BGP: Ran out of docstring while parsing '[no] distance (1-255) <A.B.C.D/M | X:X::X:X/M>$prefix [WORD$acl]'
2020/10/30 16:02:26 BGP: Ran out of docstring while parsing '[no] distance (1-255) <A.B.C.D/M | X:X::X:X/M>$prefix [WORD$acl]'
2020/10/30 16:02:26 BGP: Ran out of docstring while parsing '[no] distance (1-255) <A.B.C.D/M | X:X::X:X/M>$prefix [WORD$acl]'
2020/10/30 16:02:26 BGP: Ran out of docstring while parsing '[no] distance (1-255) <A.B.C.D/M | X:X::X:X/M>$prefix [WORD$acl]'
2020/10/30 16:02:26 BGP: Ran out of docstring while parsing '[no] distance (1-255) <A.B.C.D/M | X:X::X:X/M>$prefix [WORD$acl]'
```

Signed-off-by: Stephen Worley <sworley@nvidia.com>