[MRG] fixed failing doctest

[bthirion]

Addresses #404

[coveralls]

Coverage remained the same at 87.138% when pulling 32cb61e on bthirion:fix_mfx_doctest into f8d3149 on nipy:master.

[codecov-io]

Current coverage is 83.52% (diff: 100%)

Merging #406 into master will not change coverage

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  Hits          23185      23185          
  Misses         3570       3570          
  Partials       1002       1002          

Powered by Codecov. Last update f8d3149...32cb61e

[matthew-brett]

Sorry for my ignorance, but can you explain the change?

[bthirion]

I removed the cases where the simulated effects are 0: In these cases, due to the constraint in l.376, the mfx test can be more variable then the classical Student alternative, leading to a failure of the docstring.

[matthew-brett]

Thanks - that's helpful.