fix usage docs for "-k" (addresses issue357)
fix usage docs for "-k" (addresses issue357)

nicoddemus/repo-test,nicoddemus/pytest,rmfitzpatrick/pytest,vodik/pytest,davidszotten/pytest,pelme/pytest,hunse/pytest,abusalimov/pytest,tgoodlet/pytest,rouge8/pytest,Akasurde/pytest,doordash/pytest,untitaker/pytest,Bjwebb/pytest,untitaker/pytest,ghostsquad/pytest,vmalloc/dessert,MichaelAquilina/pytest,hunse/pytest,malinoff/pytest,Haibo-Wang-ORG/pytest,nicoddemus/pytest,nicoddemus/repo-test,omarkohl/pytest,Haibo-Wang-ORG/pytest,ghostsquad/pytest,vodik/pytest,MengJueM/pytest,Carreau/pytest,bubenkoff/pytest,wfxiang08/pytest,hackebrot/pytest,chiller/pytest,mhils/pytest,oleg-alexandrov/pytest,Bachmann1234/pytest,userzimmermann/pytest,ropez/pytest,gabrielcnr/pytest,codewarrior0/pytest,JonathonSonesen/pytest,pfctdayelise/pytest,chiller/pytest,chillbear/pytest,bukzor/pytest,wfxiang08/pytest,MengJueM/pytest,rouge8/pytest,pelme/pytest,chillbear/pytest,icemac/pytest,mdboom/pytest,markshao/pytest,ddboline/pytest,ionelmc/pytest,jb098/pytest,lukas-bednar/pytest,ionelmc/pytest,hpk42/pytest,inirudebwoy/pytest,doordash/pytest,alfredodeza/pytest,ericdill/pytest,icemac/pytest,The-Compiler/pytest,Carreau/pytest,omarkohl/pytest,takluyver/pytest,Bjwebb/pytest,RonnyPfannschmidt/pytest,ojake/pytest,mbirtwell/pytest,lukas-bednar/pytest,flub/pytest,skylarjhdownes/pytest,mdboom/pytest,The-Compiler/pytest,abusalimov/pytest,mhils/pytest,ropez/pytest,userzimmermann/pytest,jaraco/pytest,eli-b/pytest,etataurov/pytest,bubenkoff/pytest,codewarrior0/pytest,tomviner/pytest,tomviner/pytest,jb098/pytest,txomon/pytest,hpk42/pytest,oleg-alexandrov/pytest,pytest-dev/pytest,mbirtwell/pytest,tareqalayan/pytest,Bachmann1234/pytest,JonathonSonesen/pytest,takluyver/pytest,gabrielcnr/pytest,bukzor/pytest,ojake/pytest,inirudebwoy/pytest,ericdill/pytest