NSX: fix wording for configuration option

This patch clarifies that in order to use NSX distributed
routers, the replication_mode parameter should be set to
'service'. This is already its default value, so no further
change is needed beyond clarifying wording.

Change-Id: I446c428ecf8c0bf7f6947e69562d60a7fb4383ed
Closes-Bug: #1355502
Merge "NSX: fix wording for configuration option"