DOC: fix typo in cython optimize example (#15768)
DOC: fix typo in cython optimize example (#15768)

In the "full output" example, I think the return type should be `zeros_full_output`
not `brent_full_output` which is undefined

[ci skip]
mdhaber/scipy,scipy/scipy,vigna/scipy,ilayn/scipy,scipy/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,rgommers/scipy,anntzer/scipy,mdhaber/scipy,scipy/scipy,anntzer/scipy,anntzer/scipy,andyfaff/scipy,perimosocordiae/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,rgommers/scipy,anntzer/scipy,rgommers/scipy,mdhaber/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,vigna/scipy,endolith/scipy,andyfaff/scipy,anntzer/scipy,ilayn/scipy,rgommers/scipy,mdhaber/scipy,WarrenWeckesser/scipy,andyfaff/scipy,perimosocordiae/scipy,perimosocordiae/scipy,mdhaber/scipy,scipy/scipy,scipy/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,ilayn/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,ilayn/scipy,Stefan-Endres/scipy,rgommers/scipy,vigna/scipy,mdhaber/scipy,WarrenWeckesser/scipy,anntzer/scipy,perimosocordiae/scipy,endolith/scipy,endolith/scipy,andyfaff/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,scipy/scipy,vigna/scipy,ilayn/scipy,vigna/scipy,endolith/scipy,ilayn/scipy,Stefan-Endres/scipy