bug: doc str: ants fixed vs moving description

[gciccarelli3]

Summary

Doc strings for moving img and fixed img help are identical. I think only one of the images has the transformation applied to it.

Actual behavior

Expected behavior

I think the moving image is the image that is warped and the fixed image is the space to which the moving image is warped.

From pg 4 of the ants pdf doc:
"The moving image will be deformed to match the fixed image."
http://stnava.github.io/ANTsDoc/

How to replicate the behavior

See line: https://github.com/nipy/nipype/blob/master/nipype/interfaces/ants/registration.py
22 and 26

fixed_image = InputMultiPath(File(exists=True), mandatory=True,
                             desc=('image to apply transformation to (generally a coregistered '
                                   'functional)'))
moving_image = InputMultiPath(File(exists=True), argstr='%s',
                              mandatory=True,
                              desc=('image to apply transformation to (generally a coregistered '
                                    'functional)'))

Platform details:

please paste the output of: python -c "import nipype; print(nipype.get_info())"

[mgxd]

also mentioned in #1223 - I'll make a quick fix