daofind - small change in documentation

I added `()` in the explanation for the FWHM. The spaces in the formula already indicated that the entire `2.0*sqrt(2.0*log(2.0))` should be part of the denominator, but since this looks like a formula in code we should make it mathematically right by setting the required parenthses.
Merge pull request #143 from hamogu/patch-1

daofind - small change in documentation