Fix long lines in the draft

[mjethanandani]

/app/draft/draft-ietf-idr-bgp-model-18.xml(711): Warning: Too long line found (L856), 3 characters longer than 72 characters:
namespace: urn:ietf:params:xml:ns:yang:iana-bgp-community-types
/app/draft/draft-ietf-idr-bgp-model-18.xml(1871): Warning: Too long line found (L2387), 2 characters longer than 72 characters:
when "derived-from-or-self(../../name, 'bt:graceful-restart')";
/app/draft/draft-ietf-idr-bgp-model-18.xml(3093): Warning: Too long line found (L3842), 2 characters longer than 72 characters:
when "derived-from-or-self(../name, 'bt:ipv4-labeled-unicast')" {
/app/draft/draft-ietf-idr-bgp-model-18.xml(3093): Warning: Too long line found (L3854), 2 characters longer than 72 characters:
when "derived-from-or-self(../name, 'bt:ipv6-labeled-unicast')" {
/app/draft/draft-ietf-idr-bgp-model-18.xml(3093): Warning: Too long line found (L3899), 2 characters longer than 72 characters:
when "derived-from-or-self(../name, 'bt:l3vpn-ipv4-multicast')" {
/app/draft/draft-ietf-idr-bgp-model-18.xml(3093): Warning: Too long line found (L3912), 2 characters longer than 72 characters:
when "derived-from-or-self(../name, 'bt:l3vpn-ipv6-multicast')" {