fixed a typo in the help for -d option
fixed a typo in the help for -d option

jas14/khmer,F1000Research/khmer,kdmurray91/khmer,ged-lab/khmer,souravsingh/khmer,Winterflower/khmer,F1000Research/khmer,kdmurray91/khmer,kdmurray91/khmer,Winterflower/khmer,ged-lab/khmer,F1000Research/khmer,ged-lab/khmer,souravsingh/khmer,jas14/khmer,jas14/khmer,souravsingh/khmer,Winterflower/khmer