DOC: stats: fix docstring in gamma pdf (#8563)
DOC: stats: fix docstring in gamma pdf (#8563)

$\Gamma$ is more often used to denote the Gamma function instead of $\gamma$
person142/scipy,lhilt/scipy,jamestwebber/scipy,rgommers/scipy,e-q/scipy,anntzer/scipy,jamestwebber/scipy,vigna/scipy,person142/scipy,anntzer/scipy,aarchiba/scipy,aeklant/scipy,mdhaber/scipy,rgommers/scipy,jor-/scipy,grlee77/scipy,arokem/scipy,vigna/scipy,jamestwebber/scipy,matthew-brett/scipy,arokem/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,nmayorov/scipy,Eric89GXL/scipy,person142/scipy,zerothi/scipy,gfyoung/scipy,nmayorov/scipy,arokem/scipy,andyfaff/scipy,perimosocordiae/scipy,perimosocordiae/scipy,nmayorov/scipy,andyfaff/scipy,aarchiba/scipy,Eric89GXL/scipy,person142/scipy,Eric89GXL/scipy,endolith/scipy,endolith/scipy,vigna/scipy,Stefan-Endres/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,gfyoung/scipy,matthew-brett/scipy,pizzathief/scipy,tylerjereddy/scipy,pizzathief/scipy,ilayn/scipy,zerothi/scipy,scipy/scipy,aarchiba/scipy,endolith/scipy,gfyoung/scipy,matthew-brett/scipy,Stefan-Endres/scipy,pizzathief/scipy,ilayn/scipy,tylerjereddy/scipy,zerothi/scipy,endolith/scipy,ilayn/scipy,scipy/scipy,andyfaff/scipy,WarrenWeckesser/scipy,e-q/scipy,mdhaber/scipy,aeklant/scipy,gfyoung/scipy,mdhaber/scipy,e-q/scipy,aarchiba/scipy,Stefan-Endres/scipy,anntzer/scipy,anntzer/scipy,zerothi/scipy,Eric89GXL/scipy,jamestwebber/scipy,lhilt/scipy,Eric89GXL/scipy,aeklant/scipy,mdhaber/scipy,anntzer/scipy,scipy/scipy,mdhaber/scipy,grlee77/scipy,vigna/scipy,gertingold/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,lhilt/scipy,gertingold/scipy,ilayn/scipy,WarrenWeckesser/scipy,pizzathief/scipy,jor-/scipy,andyfaff/scipy,aeklant/scipy,Eric89GXL/scipy,e-q/scipy,anntzer/scipy,pizzathief/scipy,gertingold/scipy,jor-/scipy,rgommers/scipy,e-q/scipy,lhilt/scipy,Stefan-Endres/scipy,jor-/scipy,scipy/scipy,perimosocordiae/scipy,arokem/scipy,gertingold/scipy,WarrenWeckesser/scipy,andyfaff/scipy,matthew-brett/scipy,tylerjereddy/scipy,mdhaber/scipy,nmayorov/scipy,matthew-brett/scipy,endolith/scipy,nmayorov/scipy,andyfaff/scipy,arokem/scipy,scipy/scipy,person142/scipy,ilayn/scipy,zerothi/scipy,rgommers/scipy,grlee77/scipy,vigna/scipy,gertingold/scipy,jamestwebber/scipy,jor-/scipy,lhilt/scipy,scipy/scipy,ilayn/scipy,perimosocordiae/scipy,gfyoung/scipy,aarchiba/scipy,endolith/scipy,aeklant/scipy,rgommers/scipy,perimosocordiae/scipy,zerothi/scipy,grlee77/scipy,grlee77/scipy,WarrenWeckesser/scipy