corrected documentation example (&std::sqrt must be casted to be a valid functor)
corrected documentation example (&std::sqrt must be casted to be a valid functor)

dagophil/vigra,ThomasWalter/vigra,stuarteberg/vigra,stuarteberg/vigra,stuarteberg/vigra,hmeine/vigra,martinsch/vigra,dagophil/vigra,kod3r/vigra,cha007/vigra,psteinb/vigra,kod3r/vigra,dstoe/vigra,mmuellner/vigra,funkey/vigra,timoMa/vigra,dagophil/vigra,capturePointer/vigra,ThomasWalter/vigra,psteinb/vigra,cha007/vigra,capturePointer/vigra,hmeine/vigra,cha007/vigra,ThomasWalter/vigra,kod3r/vigra,xiwei-zhang/vigra,DaveRichmond-/vigra,dagophil/vigra,hmeine/vigra,kod3r/vigra,timoMa/vigra,cbecker/vigra,ThomasWalter/vigra,cha007/vigra,martinsch/vigra,cbecker/vigra,xiwei-zhang/vigra,dagophil/vigra,DaveRichmond-/vigra,kod3r/vigra,xiwei-zhang/vigra,DaveRichmond-/vigra,funkey/vigra,xiwei-zhang/vigra,cha007/vigra,psteinb/vigra,cha007/vigra,DaveRichmond-/vigra,ThomasWalter/vigra,psteinb/vigra,dstoe/vigra,ThomasWalter/vigra,xiwei-zhang/vigra,martinsch/vigra,hmeine/vigra,stuarteberg/vigra,psteinb/vigra,mmuellner/vigra,timoMa/vigra,dstoe/vigra,hmeine/vigra,dstoe/vigra,mmuellner/vigra,funkey/vigra,capturePointer/vigra,timoMa/vigra,mmuellner/vigra,cbecker/vigra,capturePointer/vigra,hmeine/vigra,capturePointer/vigra,funkey/vigra,funkey/vigra,kod3r/vigra,psteinb/vigra,stuarteberg/vigra,psteinb/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,DaveRichmond-/vigra,timoMa/vigra,martinsch/vigra,martinsch/vigra,capturePointer/vigra,DaveRichmond-/vigra,xiwei-zhang/vigra,mmuellner/vigra,dstoe/vigra,mmuellner/vigra,kod3r/vigra,timoMa/vigra,cbecker/vigra,ThomasWalter/vigra,cbecker/vigra,timoMa/vigra,DaveRichmond-/vigra,stuarteberg/vigra,capturePointer/vigra,hmeine/vigra,dstoe/vigra,funkey/vigra,xiwei-zhang/vigra,stuarteberg/vigra,martinsch/vigra,funkey/vigra,cha007/vigra,cbecker/vigra,mmuellner/vigra