FIX: argstr for mask in BlurToFWHM

[mvdoc]

Fix argstr for mask input in BlurToFWHM

[effigies]

This appears to be changing the API, since -blurmaster is a valid argument. It's possible someone depends on this misnaming.

Maybe a compromise is to deprecate mask, add mask_file to correspond to -mask, and a blurmaster_file to replace the old mask.

[mvdoc]

I'm not sure I follow, because -blurmaster is another argument already present in the inputspec. Right now if somebody specifies both a blurmaster and a mask, it will fail, because it will be converted as -blurmaster blurmaster.nii.gz -blurmaster mask.nii.gz. Besides, 3dBlurToFWHM would use the smoothness of the mask as target smoothness, and if it's a binary mask that wouldn't work.

[effigies]

Oh, sorry. I didn't read the context and thought you were replacing blurmaster with mask. Yes, this looks like a copy-paste job that just forgot to change the argstr.

LGTM.