nodes: fix string possibly stored in Node.keywords instead of MarkDecorator
nodes: fix string possibly stored in Node.keywords instead of MarkDecorator

This mistake was introduced in 7259c453d6c1dba6727cd328e6db5635ccf5821c.

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