Clarify that ${name} is required for replication.

The documentation now makes it clear that the ${name} wildcard is
required for gerrit to be able to match URLs to the projects that
it needs to replicate.

Bug: issue 896
Change-Id: I51a6ea29ced0b6bc782467692bfb22fe9a706207
Signed-off-by: Francois Marier <francois@debian.org>