RF: pep8 doctest in Formula subtraction
RF: pep8 doctest in Formula subtraction

Put more spaces around to make it easier to read.

alexis-roche/nipy,alexis-roche/nipy,alexis-roche/nipy,alexis-roche/nireg,alexis-roche/nipy,alexis-roche/nireg,nipy/nireg,nipy/nireg