Remove typo in README.markdown.
Remove typo in README.markdown.

GuillaumeDievart/atoum,CedCannes/atoum,agallou/atoum,jubianchi/atoum,mageekguy/atoum,vonglasow/atoum,vonglasow/atoum,gpaton/atoum,agallou/atoum,jubianchi/atoum,tejerka/atoum,Tiross/atoum,mikaelrandy/atoum,gpaton/atoum,tejerka/atoum,mikaelrandy/atoum,Tiross/atoum,GuillaumeDievart/atoum,mageekguy/atoum,CedCannes/atoum