Minor clean-up of the FWHM to sigma conversion

No need to use 'fwhm' on both sides of the = sign and the comment about
not using /= with numpy scalar wasn't entirely helping.
Merge pull request #283 from lesteve/fwhm-to-sigma-conversion-tweak

[MRG] Minor clean-up of the FWHM to sigma conversion
Merge branch 'master' of https://github.com/dohmatob/nilearn