Add force-draft argument description

Add metaVar so the user gets a proper message when using the command
without the required argument.

'fatal: Argument "{CHANGE,PATCHSET}" is required'

instead of

'fatal: Argument "" is required'

Change-Id: I3f56dd7d4613e2ff9dbbc78a7a82452c969170b7
diff --git a/src/main/java/com/googlesource/gerrit/plugins/forcedraft/ForceDraft.java b/src/main/java/com/googlesource/gerrit/plugins/forcedraft/ForceDraft.java
index 712d587..7840751 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/forcedraft/ForceDraft.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/forcedraft/ForceDraft.java
@@ -54,7 +54,8 @@
   @Inject
   private @GerritServerConfig Config config;
 
-  @Argument(index = 0, required = true, usage = "<change, patch set> to be changed to draft")
+  @Argument(index = 0, required = true, metaVar = "{CHANGE,PATCHSET}",
+      usage = "<change, patch set> to be changed to draft")
   private void addPatchSetId(final String token) {
     try {
       patchSet = parsePatchSet(token);