Remove double forall from "repo help forall" output

%prog represents the full subcommand ("repo" + subcommand name), not a
Windows-style environment variable for "repo". The current help output
shows

 repo forall% forall ...

Correct the variable usage so it shows "repo forall ..." instead.

Change-Id: I1fea55572428cc922ddf24ace1168a3d8f82dad0

subcmds/forall.py

@@ -105,7 +105,7 @@ following <command>.
 
 Example: to list projects:
 
-  %prog% forall -c 'echo $REPO_PROJECT'
+  %prog -c 'echo $REPO_PROJECT'
 
 Notice that $REPO_PROJECT is quoted to ensure it is expanded in
 the context of running <command> instead of in the calling shell.