Quick doc fix on xfail reason parameter
Quick doc fix on xfail reason parameter

As per https://github.com/pytest-dev/pytest/pull/6957/files#r399564043

nicoddemus/pytest,RonnyPfannschmidt/pytest,pytest-dev/pytest,The-Compiler/pytest,nicoddemus/pytest,Akasurde/pytest,alfredodeza/pytest,markshao/pytest,The-Compiler/pytest