Correct C changelog for extend_haplotypes

[benjeffery]

Reviewing before release I spotted that the method had changed name without the changelog being updated. Hopefully I have this right @petrelharp?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (6cc7ddd) to head (a56df1a).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3013      +/-   ##
==========================================
+ Coverage   89.80%   89.82%   +0.01%     
==========================================
  Files          29       29              
  Lines       31979    32030      +51     
  Branches     6190     6207      +17     
==========================================
+ Hits        28720    28772      +52     
+ Misses       1861     1859       -2     
- Partials     1398     1399       +1     

Flag	Coverage Δ
c-tests	86.69% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	89.05% <ø> (ø)
python-tests	99.02% <ø> (+<0.01%)	⬆️

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see 3 files with indirect coverage changes

[petrelharp]

Good catch, thanks!