src/app/libmain.cxx: correct output of rpm-ostree compose -h

Need append a space between container-encapsulate and Generate in
rpm-ostree compose -h
...
$ rpm-ostree compose -h
[root@localhost ~]# rpm-ostree compose -h
Usage:
  rpm-ostree compose [OPTION…] COMMAND

Commands to compose a tree

Builtin "compose" Commands:
  commit           Commit a target path to an OSTree repository
  container-encapsulateGenerate a reproducible "chunked" container image(using RPM data) from an OSTree commit
...

It also corrected the bash completion of `rpm-ostree compose'
...
[root@localhost ~]# rpm-ostree compose
--help                         image
--quiet                        install
commit                         postprocess
container-encapsulateGenerate  tree
extensions
[root@localhost ~]# rpm-ostree compose
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>

src/app/libmain.cxx

@@ -209,7 +209,7 @@ option_context_new_with_commands (RpmOstreeCommandInvocation *invocation,
       gboolean hidden = (command->flags & RPM_OSTREE_BUILTIN_FLAG_HIDDEN) > 0;
       if (!hidden)
         {
-          g_string_append_printf (summary, "\n  %-17s", command->name);
+          g_string_append_printf (summary, "\n  %-23s", command->name);
           if (command->description != NULL)
             g_string_append_printf (summary, "%s", command->description);
         }