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