Fix learner for basics of DML in R

### 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_