CONTRIBUTING.rst: spelling fixes
CONTRIBUTING.rst: spelling fixes

[ci skip]

alfredodeza/pytest,hpk42/pytest,Akasurde/pytest,skylarjhdownes/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,vmalloc/dessert,tomviner/pytest,pfctdayelise/pytest,ddboline/pytest,tgoodlet/pytest,pytest-dev/pytest,davidszotten/pytest,hackebrot/pytest,markshao/pytest,jaraco/pytest,malinoff/pytest,rmfitzpatrick/pytest,tomviner/pytest,The-Compiler/pytest,hpk42/pytest,tareqalayan/pytest,flub/pytest,The-Compiler/pytest,eli-b/pytest,JonathonSonesen/pytest,MichaelAquilina/pytest,etataurov/pytest,nicoddemus/pytest,JonathonSonesen/pytest,txomon/pytest