Fix usage for -m siena_diff option

Presently the -m option for siena_diff has usage that says:

[-m]            apply <input1_basename>_stdmask to brain edge points

However, in siena_diff there is no reference to standard space at all. The -m option sets edge_masking that then activates resampling of <input1>_valid_mask_with_<input2> into <input1>_halfwayto_<input2>_valid_mask, applying that mask in later computations.

Hence, this PR simply updates the -m usage to remove reference to stdmask so that it instead reads:

[-m]            apply <input1_basename>_valid_mask_with_<input2_basename> to brain edge points

CC: @mark