Fix snapshot-create-as syntax in help output

Fix snapshot-create-as syntax in help output

According to the man page, the memspec parameter should have the
'--memspec' option mandatory and this is as close as we can get to
that.  What this change does is explained below.

man virsh:

snapshot-create-as ... [[--live] [--memspec memspec]]

virsh help snapshot-create-as before this patch:

  SYNOPSIS
    snapshot-create-as ... [<memspec>] ...
...
  OPTIONS
    [--memspec] <string> ...

virsh help snapshot-create-as after this patch:

  SYNOPSIS
    snapshot-create-as ... [--memspec <string>] ...
...
  OPTIONS
    --memspec <string> ...