Yep the statistic for grassland covering one-fifth of the earth looks inaccurate. It's more like two-fifths. Here are a few sources -

http://www.wri.org/publication/content/8269

http://www.pbs.org/journeytoplanetearth/stateoftheplanet/grasslands.html

Africa is covered nearly 50% by grasslands. Just because it's lesser in a few countries does not mean the world percentage will be vastly effected.