Improve the wording to explain url matching

Everytime I use this switch I struggle and end up having to look a the
code to figure out how it works. Help clarify how the match works.

Change-Id: I1001e83d677483bf8372febc4cc6f3eea15b8341

src/main/java/com/googlesource/gerrit/plugins/replication/StartCommand.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/StartCommand.java b/src/main/java/com/googlesource/gerrit/plugins/replication/StartCommand.java
index f63df98..800245e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/StartCommand.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/StartCommand.java
@@ -37,7 +37,7 @@
   @Option(name = "--all", usage = "push all known projects")
   private boolean all;
 
-  @Option(name = "--url", metaVar = "PATTERN", usage = "pattern to match URL on")
+  @Option(name = "--url", metaVar = "SUBSTRING", usage = "substring URL must match (or * to match everything)")
   private String urlMatch;
 
   @Option(name = "--wait", usage = "wait for replication to finish before exiting")