Fix formatting in DataFrame documentation and add example usage for Arrow integration

Author: kosiew

docs/source/user-guide/dataframe/index.rst

@@ -179,7 +179,7 @@ PyArrow
 
 DataFrames are also iterable, yielding :class:`datafusion.RecordBatch`
 objects lazily so you can loop over results directly without importing
-PyArrow::
+PyArrow:
 
 .. code-block:: python
 

docs/source/user-guide/io/arrow.rst

@@ -74,6 +74,8 @@ Python library that accepts these can import a DataFusion DataFrame directly.
 
 .. ipython:: python
 
+    from datafusion import col, lit
+
     df = df.select((col("a") * lit(1.5)).alias("c"), lit("df").alias("d"))
     pa.table(df)