[SPARK-45611][PYTHON][DOCS] Typo fixed yyy to yyyy at date_format fun…

…ction

Based on the issue https://issues.apache.org/jira/browse/SPARK-45611

### What changes were proposed in this pull request?
In the `date_format` method's doctest, there is a typo in the year format. Instead of '`MM/dd/yyy'`, it should be `'MM/dd/yyyy'` as the expected output`[Row(date='04/08/2015')]` indicates the following format `"MM/dd/yyyy"`

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #43442 from metecanakar/fix/typo-in-data-format.

Lead-authored-by: Mete Can Akar <metecanakar@gmail.com>
Co-authored-by: Mete Can Akar <10785597+metecanakar@users.noreply.github.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/sql/functions.py

@@ -6784,7 +6784,7 @@ def date_format(date: "ColumnOrName", format: str) -> Column:
     Examples
     --------
     >>> df = spark.createDataFrame([('2015-04-08',)], ['dt'])
-    >>> df.select(date_format('dt', 'MM/dd/yyy').alias('date')).collect()
+    >>> df.select(date_format('dt', 'MM/dd/yyyy').alias('date')).collect()
     [Row(date='04/08/2015')]
     """
     return _invoke_function("date_format", _to_java_column(date), format)