Fix typo in data preparation script, add reference.
Fix typo in data preparation script, add reference.

The `_sobol_direction_numbers` file was generated properly, it was just a typo in the script.

scipy/scipy,anntzer/scipy,Eric89GXL/scipy,vigna/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,matthew-brett/scipy,vigna/scipy,zerothi/scipy,Eric89GXL/scipy,endolith/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,ilayn/scipy,e-q/scipy,mdhaber/scipy,anntzer/scipy,endolith/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,tylerjereddy/scipy,scipy/scipy,andyfaff/scipy,andyfaff/scipy,vigna/scipy,andyfaff/scipy,matthew-brett/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,rgommers/scipy,mdhaber/scipy,endolith/scipy,Stefan-Endres/scipy,scipy/scipy,perimosocordiae/scipy,andyfaff/scipy,tylerjereddy/scipy,anntzer/scipy,andyfaff/scipy,grlee77/scipy,rgommers/scipy,Eric89GXL/scipy,scipy/scipy,grlee77/scipy,endolith/scipy,ilayn/scipy,e-q/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,zerothi/scipy,e-q/scipy,ilayn/scipy,grlee77/scipy,Stefan-Endres/scipy,zerothi/scipy,zerothi/scipy,matthew-brett/scipy,Stefan-Endres/scipy,mdhaber/scipy,scipy/scipy,ilayn/scipy,endolith/scipy,anntzer/scipy,tylerjereddy/scipy,ilayn/scipy,WarrenWeckesser/scipy,mdhaber/scipy,perimosocordiae/scipy,endolith/scipy,grlee77/scipy,perimosocordiae/scipy,anntzer/scipy,tylerjereddy/scipy,ilayn/scipy,e-q/scipy,WarrenWeckesser/scipy,rgommers/scipy,matthew-brett/scipy,matthew-brett/scipy,andyfaff/scipy,tylerjereddy/scipy,Eric89GXL/scipy,scipy/scipy,mdhaber/scipy,mdhaber/scipy,zerothi/scipy,vigna/scipy,e-q/scipy,zerothi/scipy,anntzer/scipy,vigna/scipy,grlee77/scipy,WarrenWeckesser/scipy,rgommers/scipy,rgommers/scipy