docs(python): Minor tweak in code example in section Expressions/Missing data

[jrycw]

It can be observed that the output format of fill_literal_df is not consistent with other examples in this section. This inconsistency is due to the presence of an extra comma. Instead of simply removing the extra comma, I would suggest rewriting the code as proposed, which aligns with the format of other code examples in this section.

fill_literal_df = (
    df.with_columns(
        pl.col("col2").fill_null(
            pl.lit(2),
        ),
    ), # <= extra comma
)
print(fill_literal_df)

(shape: (3, 2)
┌──────┬──────┐
│ col1 ┆ col2 │
│ ---  ┆ ---  │
│ i64  ┆ i64  │
╞══════╪══════╡
│ 1    ┆ 1    │
│ 2    ┆ 2    │
│ 3    ┆ 3    │
└──────┴──────┘,)

[stinodego]

Yep, thanks!