Fix a namespace nit in folly/futures/Future.cpp.
Fix a namespace nit in folly/futures/Future.cpp.

Summary: [Folly] Fix a namespace nit in folly/futures/Future.cpp.

As soon as someone introduces a namespace `folly::futures::detail` this will blow up. Fix it so it won't blow up when that happens.

Reviewed By: @​hannesr

Differential Revision: D2258799

mqeizi/folly,alexst07/folly,tempbottle/folly,tempbottle/folly,tempbottle/folly,clearlylin/folly,guker/folly,Orvid/folly,tomhughes/folly,loversInJapan/folly,kernelim/folly,brunomorishita/folly,constantine001/folly,xzmagic/folly,KSreeHarsha/folly,raphaelamorim/folly,sakishum/folly,mqeizi/folly,loversInJapan/folly,PoisonBOx/folly,alexst07/folly,gavioto/folly,kernelim/folly,clearlylin/folly,clearlylin/folly,juniway/folly,facebook/folly,sakishum/folly,chjp2046/folly,constantine001/folly,KSreeHarsha/folly,hongliangzhao/folly,raphaelamorim/folly,constantine001/folly,charsyam/folly,gaoyingie/folly,bobegir/folly,tomhughes/folly,chjp2046/folly,CJstar/folly,sakishum/folly,xzmagic/folly,loversInJapan/folly,wildinto/folly,guker/folly,arg0/folly,yangjin-unique/folly,cole14/folly,tomhughes/folly,Hincoin/folly,rklabs/folly,sakishum/folly,mqeizi/folly,bowlofstew/folly,bowlofstew/folly,doctaweeks/folly,charsyam/folly,yangjin-unique/folly,cole14/folly,PoisonBOx/folly,gavioto/folly,stonegithubs/folly,juniway/folly,bowlofstew/folly,hongliangzhao/folly,brunomorishita/folly,lifei/folly,upsoft/folly,kernelim/folly,renyinew/folly,CJstar/folly,zhiweicai/folly,clearlylin/folly,xzmagic/folly,Hincoin/folly,Eagle-X/folly,reddit/folly,PoisonBOx/folly,renyinew/folly,bikong2/folly,bikong2/folly,constantine001/folly,wildinto/folly,alexst07/folly,lifei/folly,juniway/folly,PPC64/folly,arg0/folly,Eagle-X/folly,tomhughes/folly,charsyam/folly,PPC64/folly,gavioto/folly,facebook/folly,upsoft/folly,bowlofstew/folly,loversInJapan/folly,lifei/folly,raphaelamorim/folly,chjp2046/folly,juniway/folly,SammyK/folly,rklabs/folly,Eagle-X/folly,guker/folly,yangjin-unique/folly,brunomorishita/folly,leolujuyi/folly,cole14/folly,SammyK/folly,yangjin-unique/folly,guker/folly,juniway/folly,KSreeHarsha/folly,upsoft/folly,floxard/folly,doctaweeks/folly,yangjin-unique/folly,zhiweicai/folly,stonegithubs/folly,floxard/folly,facebook/folly,raphaelamorim/folly,Hincoin/folly,mqeizi/folly,sakishum/folly,KSreeHarsha/folly,floxard/folly,fw1121/folly,sonnyhu/folly,guker/folly,CJstar/folly,rklabs/folly,gaoyingie/folly,doctaweeks/folly,Orvid/folly,zhiweicai/folly,leolujuyi/folly,SammyK/folly,Hincoin/folly,KSreeHarsha/folly,brunomorishita/folly,Eagle-X/folly,SeanRBurton/folly,shaobz/folly,CJstar/folly,chjp2046/folly,upsoft/folly,kernelim/folly,PoisonBOx/folly,alexst07/folly,reddit/folly,arg0/folly,theiver9827/folly,renyinew/folly,bikong2/folly,reddit/folly,chjp2046/folly,reddit/folly,PoisonBOx/folly,rklabs/folly,SeanRBurton/folly,theiver9827/folly,sonnyhu/folly,bobegir/folly,SeanRBurton/folly,renyinew/folly,doctaweeks/folly,sonnyhu/folly,brunomorishita/folly,cole14/folly,raphaelamorim/folly,facebook/folly,shaobz/folly,wildinto/folly,hongliangzhao/folly,zhiweicai/folly,floxard/folly,PPC64/folly,shaobz/folly,clearlylin/folly,renyinew/folly,Orvid/folly,xzmagic/folly,SeanRBurton/folly,fw1121/folly,tomhughes/folly,charsyam/folly,leolujuyi/folly,sonnyhu/folly,facebook/folly,wildinto/folly,Hincoin/folly,bikong2/folly,rklabs/folly,zhiweicai/folly,tempbottle/folly,reddit/folly,bobegir/folly,shaobz/folly,alexst07/folly,gaoyingie/folly,fw1121/folly,SeanRBurton/folly,theiver9827/folly,leolujuyi/folly,arg0/folly,fw1121/folly,tempbottle/folly,stonegithubs/folly,theiver9827/folly,hongliangzhao/folly,CJstar/folly,gavioto/folly,Orvid/folly,loversInJapan/folly,Orvid/folly,wildinto/folly,constantine001/folly,doctaweeks/folly,mqeizi/folly,gaoyingie/folly,upsoft/folly,Eagle-X/folly,sonnyhu/folly,hongliangzhao/folly,lifei/folly,SammyK/folly,stonegithubs/folly,xzmagic/folly,kernelim/folly,bikong2/folly,gaoyingie/folly,stonegithubs/folly,gavioto/folly,shaobz/folly,fw1121/folly,bobegir/folly,leolujuyi/folly,bowlofstew/folly,arg0/folly,floxard/folly,cole14/folly,bobegir/folly,lifei/folly,PPC64/folly,SammyK/folly,PPC64/folly,theiver9827/folly