Erroneous causal diagram in PLIV Documentation

### Description of the issue or change proposal

https://docs.doubleml.org/stable/guide/models.html#partially-linear-iv-regression-model-pliv

There seems to be an accidental bidirected edge between the `exposure` variable and `outcome` variable in this causal diagram [here](https://docs.doubleml.org/stable/guide/models.html#id22) for section 3.1.4. Partially linear IV regression model (PLIV)

Is this correct?

### Comments, context or references

_No response_