Fix error in gather_if's documentation
Fix error in gather_if's documentation

Reported by Björn Hegstam

GrimDerp/thrust,jaredhoberock/thrust,xiongzhanblake/thrust_src,thvasilo/thrust,raygit/thrust,zhenglaizhang/thrust,thrust/thrust,sdalton1/thrust,egaburov/thrust,Ricardo666666/thrust,marksantos/thrust,zeryx/thrust,arnabgho/thrust,andrewcorrigan/thrust-multi-permutation-iterator,thvasilo/thrust,andrewcorrigan/thrust-multi-permutation-iterator,sarvex/thrust,zhenglaizhang/thrust,dachziegel/thrust,thrust/thrust,xiongzhanblake/thrust,mohamed-ali/thrust,dachziegel/thrust,jaredhoberock/thrust,thrust/thrust,zeryx/thrust,sarvex/thrust,jaredhoberock/thrust,zeryx/thrust,thvasilo/thrust,thrust/thrust,sdalton1/thrust,marksantos/thrust,xiongzhanblake/thrust_src,mohamed-ali/thrust,mohamed-ali/thrust,Ricardo666666/thrust,xiongzhanblake/thrust,dachziegel/thrust,egaburov/thrust,jaredhoberock/thrust,arnabgho/thrust,zhenglaizhang/thrust,raygit/thrust,GrimDerp/thrust,marksantos/thrust,arnabgho/thrust,GrimDerp/thrust,Ricardo666666/thrust,sdalton1/thrust,egaburov/thrust,sarvex/thrust,raygit/thrust,xiongzhanblake/thrust,andrewcorrigan/thrust-multi-permutation-iterator,thrust/thrust,xiongzhanblake/thrust_src,jaredhoberock/thrust