Minor grammar fixes in pytest.mark.xfail docs
Minor grammar fixes in pytest.mark.xfail docs

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