fixed_image and moving_image have same description in ants.Registration docstring

[dohmatob]

https://github.com/nipy/nipype/blob/master/nipype/interfaces/ants/registration.py#L21
In the documentation of the fixed_image param of ants.Registration, it's said "... image to which transformation will be applied..". I think this should be "... template / target image ...". In fact, it seems the fixed_image and moving_image params were mistakenly giving exactly the same documentation in the doctstring.

[satra]

@dohmatob - thanks for the check will update shortly.

[dohmatob]

OK @satra.