Fix error in Heip E index computation

I believe that the computation of Heip's evenness index is wrong.
In Heip and Engels, J.mar.biol.Ass.UK 54:559-563 (1974) the formula is E = ((e^H) -1) / (S-1).
Instead, here it is computed as e^(H -1) / (S-1).

I noticed this while comparing the heip_e results obtained by Qiime vs. Mothur - which reports results consistent with my correction.