plot_haplotypes_frequencies.ipynb: ag3 used instead of af1 in Af1 section

[star-lord1234]

Bug description:

In notebooks/plot_haplotypes_frequencies.ipynb, the Af1 section computes haplotype frequencies using af1, but the final plotting call is made on ag3.

hap_xr = af1.haplotypes_frequencies_advanced(
region="2RL:2,358,158-2,431,617",
area_by="admin1_iso",
period_by="year",
sample_sets=["1232-VO-KE-OCHOMO-VMF00044"],
min_cohort_size=10,
)

ag3.plot_frequencies_time_series(hap_xr)
Expected behaviour

The plotting call should use the same dataset instance (af1) that was used to compute the haplotypes:

af1.plot_frequencies_time_series(hap_xr)
Impact

After inspecting the implementation of plot_frequencies_time_series in
malariagen_data/anoph/frq_base.py, the current code still produces the correct output because the function operates entirely on the dataset argument (ds) and does not rely on instance-specific state.

However, the mismatch may still cause confusion because:

It appears inconsistent within an otherwise af1-only section of the notebook

Readers may assume the wrong API client is intended

It could lead to incorrect copy-paste usage in contexts where instance state does matter

Proposed fix

Change the final line of the Af1 section from:

ag3.plot_frequencies_time_series(hap_xr)

to:

af1.plot_frequencies_time_series(hap_xr)

Simple change that does not change behaiviour for user convenience