fix broken ectj link

Author: SvenKlaassen

doc/conf.py

@@ -129,6 +129,7 @@
     'https://doi.org/10.1111/ectj.12097',  # Valid DOI; Causes 403 Client Error: Forbidden for url:...
     'https://doi.org/10.2307/2171802',  # Valid DOI; Causes 403 Client Error: Forbidden for url:...
     'https://doi.org/10.2307/1912705',  # Valid DOI; Causes 403 Client Error: Forbidden for url:...
+    'https://doi.org/10.1093/ectj/utaa027', # Valid DOI; Causes 403 Client Error: Forbidden for url:...
 ]
 
 # To execute R code via jupyter-execute one needs to install the R kernel for jupyter

doc/examples/py_double_ml_cate.ipynb

@@ -254,7 +254,7 @@
     }
    },
    "source": [
-    "To estimate the CATE, we rely on the best-linear-predictor of the linear score as in [Semenova et al.](https://academic.oup.com/ectj/article/24/2/264/5899048) To approximate the target function $g(x)$ with a linear form, we have to define a data frame of basis functions. Here, we rely on [patsy](https://patsy.readthedocs.io/en/latest/) to construct a suitable basis of [B-splines](https://en.wikipedia.org/wiki/B-spline)."
+    "To estimate the CATE, we rely on the best-linear-predictor of the linear score as in [Semenova et al.](https://doi.org/10.1093/ectj/utaa027) To approximate the target function $g(x)$ with a linear form, we have to define a data frame of basis functions. Here, we rely on [patsy](https://patsy.readthedocs.io/en/latest/) to construct a suitable basis of [B-splines](https://en.wikipedia.org/wiki/B-spline)."
    ]
   },
   {