[MINOR][DOCS] Updated the docstring of DataStreamWriter.foreach() method

Fixed the documentation

### What changes were proposed in this pull request?
There isn't much clarity on the docstring of the DataStreamWriter.foreach() method that takes an object with process() method, as a input. As part of this PR, The docstring is fixed.

### Why are the changes needed?
There isn't much clarity on the docstring of the DataStreamWriter.foreach() method that takes an object with process() method, as a input.

### Does this PR introduce _any_ user-facing change?
Yes. This change modifies the docstring in the DataStreamWriter.foreach() method.

### How was this patch tested?
Simple change on docstring. I haven't tested the changes. Need some support with the testing.

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

Closes #51316 from nagaarjun-p/master.

Authored-by: Mine4106 <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: nagaarjun-p

python/pyspark/sql/streaming/readwriter.py

@@ -1549,7 +1549,7 @@ def foreach(self, f: Union[Callable[[Row], None], "SupportsProcess"]) -> "DataSt
         ...     def close(self, error):
         ...         print("Closed with error: %s" % str(error))
         ...
-        >>> q = df.writeStream.foreach(print_row).start()
+        >>> q = df.writeStream.foreach(RowPrinter()).start()
         >>> time.sleep(3)
         >>> q.stop()
         """