T1_mask needs to be a brain-extracted T1

[mrribbits]

Just quick a minor suggestion for clarifying the documentation a tiny bit: the "T1_mask" input should be a brain-extracted T1 image and not a binary mask image. FAST kept failing on me during the epi_reg step until I realized this. Thx!

[hanscol]

Thanks for the heads up, I updated the documentation to clarify this.