update example to polars 0.30.0

[wukan1986]

error[E0271]: type mismatch resolving <Vec<DataFrame> as IntoIterator>::Item == DataFrame --> src/parallel_jaccard_mod.rs:67:36	67	accumulate_dataframes_vertical(dfs)	------------------------------ ^^^ expected DataFrame, found polars::prelude::DataFrame
required by a bound introduced by this call

= note: `polars::prelude::DataFrame` and `DataFrame` have similar names, but are actually distinct types

note: polars::prelude::DataFrame is defined in crate polars_core --> /home/kan/.cargo/registry/src/github.com-1ecc6299db9ec823/polars-core-0.30.0/src/frame/mod.rs:142:1	142	pub struct DataFrame {	^^^^^^^^^^^^^^^^^^^^ note: DataFrame is defined in crate polars_core --> /home/kan/.cargo/registry/src/github.com-1ecc6299db9ec823/polars-core-0.29.0/src/frame/mod.rs:143:1
143	pub struct DataFrame {
^^^^^^^^^^^^^^^^^^^^

= note: perhaps two different versions of crate `polars_core` are being used?

note: required by a bound in accumulate_dataframes_vertical --> /home/kan/.cargo/registry/src/github.com-1ecc6299db9ec823/polars-core-0.29.0/src/utils/mod.rs:556:21 | 556 | I: IntoIterator, | ^^^^^^^^^^^^^^^^ required by this bound in accumulate_dataframes_vertical