fix wrong visibility modifier

[zh217]

This fixes the following error when using this crate:

error[E0446]: crate-private type `HmmContext` in public interface
   --> /Users/zh217/.cargo/registry/src/index.crates.io-6f17d22bba15001f/jieba-rs-0.7.0/src/hmm.rs:154:1
    |
59  | pub(crate) struct HmmContext {
    | ---------------------------- `HmmContext` declared as crate-private
...
154 | pub fn cut_internal<'a>(sentence: &'a str, words: &mut Vec<&'a str>, hmm_context: &mut HmmContext) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak crate-private type

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.62%. Comparing base (2736f41) to head (a4aee1c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   84.62%   84.62%           
=======================================
  Files           8        8           
  Lines        2036     2036           
=======================================
  Hits         1723     1723           
  Misses        313      313           

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[codspeed-hq]

CodSpeed Performance Report

Merging #113 will not alter performance

_{Comparing zh217:main (a4aee1c) with main (2736f41)}

Summary

✅ 10 untouched benchmarks