Fix learner for basics of DML in R

[PhilippBach]

Description of the issue or change proposal

Check and fix the learner in the R example on the basics of DML (partialling out score)

https://docs.doubleml.org/stable/examples/R_double_ml_basics.html#Partialling-out-score

Initially spotted by @dandls 🙏

Same issue seems to show up for Python: https://docs.doubleml.org/stable/examples/py_double_ml_basics.html#Partialling-out-score

Comments, context or references

No response

[PhilippBach]

Maybe there was a change in the learner or we'd better switch to ranger

[SvenKlaassen]

Ok, we need to check both notebooks in detail.
I guess changing to a boosting algorithm like xgboost might also reduce the simulation time.