[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 <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: vadim

python/pyspark/sql/functions.py

@@ -1716,6 +1716,8 @@ def when(condition: Column, value: Any) -> Column:
     value :
         a literal value, or a :class:`~pyspark.sql.Column` expression.
 
+    Examples
+    --------
     >>> df.select(when(df['age'] == 2, 3).otherwise(4).alias("age")).collect()
     [Row(age=3), Row(age=4)]