[SPARK-39032][PYTHON][DOCS] Examples' tag for pyspark.sql.functions.when()
[SPARK-39032][PYTHON][DOCS] Examples' tag for pyspark.sql.functions.when()

### What changes were proposed in this pull request?
Fix missing keyword for `pyspark.sql.functions.when()` documentation.

### Why are the changes needed?
[Documentation](https://spark.apache.org/docs/latest/api/python/reference/api/pyspark.sql.functions.when.html) is not formatted correctly

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
All tests passed.

Closes #36369 from vadim/SPARK-39032.

Authored-by: vadim <750346cfc72db39e91a7cc8bed3d4dfd816d0d17@users.noreply.github.com>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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