Bump custom/plugins/zsh-history-substring-search

Bumps [custom/plugins/zsh-history-substring-search](https://github.com/zsh-users/zsh-history-substring-search) from `19b8876` to `8f5d8a5`.
- [Release notes](https://github.com/zsh-users/zsh-history-substring-search/releases)
- [Commits](zsh-users/zsh-history-substring-search@19b8876...8f5d8a5)

---
updated-dependencies:
- dependency-name: custom/plugins/zsh-history-substring-search
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>