DOC: special: small fixes to incomplete gamma function docs
DOC: special: small fixes to incomplete gamma function docs

- Add missing period in an example
- Use `dlmf` for citations of the DLMF

Stefan-Endres/scipy,jor-/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,ilayn/scipy,Eric89GXL/scipy,zerothi/scipy,ilayn/scipy,andyfaff/scipy,jor-/scipy,rgommers/scipy,matthew-brett/scipy,tylerjereddy/scipy,perimosocordiae/scipy,andyfaff/scipy,rgommers/scipy,e-q/scipy,aeklant/scipy,arokem/scipy,mdhaber/scipy,matthew-brett/scipy,gertingold/scipy,anntzer/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,pizzathief/scipy,zerothi/scipy,anntzer/scipy,Stefan-Endres/scipy,mdhaber/scipy,perimosocordiae/scipy,vigna/scipy,mdhaber/scipy,jamestwebber/scipy,rgommers/scipy,tylerjereddy/scipy,anntzer/scipy,zerothi/scipy,Stefan-Endres/scipy,matthew-brett/scipy,ilayn/scipy,endolith/scipy,mdhaber/scipy,aarchiba/scipy,Eric89GXL/scipy,WarrenWeckesser/scipy,pizzathief/scipy,tylerjereddy/scipy,aeklant/scipy,jamestwebber/scipy,andyfaff/scipy,e-q/scipy,scipy/scipy,nmayorov/scipy,zerothi/scipy,nmayorov/scipy,endolith/scipy,jamestwebber/scipy,e-q/scipy,jor-/scipy,scipy/scipy,pizzathief/scipy,zerothi/scipy,gertingold/scipy,andyfaff/scipy,endolith/scipy,jamestwebber/scipy,jamestwebber/scipy,pizzathief/scipy,mdhaber/scipy,endolith/scipy,jor-/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,arokem/scipy,scipy/scipy,nmayorov/scipy,ilayn/scipy,arokem/scipy,anntzer/scipy,Eric89GXL/scipy,scipy/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,aarchiba/scipy,aarchiba/scipy,e-q/scipy,endolith/scipy,nmayorov/scipy,Eric89GXL/scipy,grlee77/scipy,vigna/scipy,andyfaff/scipy,tylerjereddy/scipy,endolith/scipy,grlee77/scipy,pizzathief/scipy,Stefan-Endres/scipy,jor-/scipy,rgommers/scipy,person142/scipy,e-q/scipy,gertingold/scipy,grlee77/scipy,zerothi/scipy,Eric89GXL/scipy,person142/scipy,aarchiba/scipy,scipy/scipy,matthew-brett/scipy,person142/scipy,ilayn/scipy,vigna/scipy,ilayn/scipy,perimosocordiae/scipy,vigna/scipy,grlee77/scipy,rgommers/scipy,person142/scipy,grlee77/scipy,scipy/scipy,aeklant/scipy,anntzer/scipy,mdhaber/scipy,nmayorov/scipy,arokem/scipy,Stefan-Endres/scipy,gertingold/scipy,gertingold/scipy,vigna/scipy,arokem/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,matthew-brett/scipy,anntzer/scipy,aeklant/scipy,aeklant/scipy,andyfaff/scipy,person142/scipy,aarchiba/scipy