Fix: Update 'zh' to 'zh-CN' in lazygit Language Configuration

[try-to-fly]

• PR Description
  This pull request corrects the language code in lazygit's documentation from 'zh' to 'zh-CN'. The previous 'zh' code was not working properly. This change improves the localization and usability for Chinese users. The change has been validated to ensure the 'zh-CN' option functions as expected.
• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go run scripts/cheatsheet/main.go generate)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[Dragon1573]

Related bug

Unable to launch lazygit when setting gui.language to 'zh'. Switching it to auto DOES fix the problem and Simplified Chinese is selected as expected.

English en, Polish pl， Nederlands nl, Japanese ja, Korean ko works as usual.

23:03:54 ~/.c/lazygit 2ms $ js-yaml config.yml | jq '.gui.language'
"zh"
23:03:58 ~/.c/lazygit 123ms $ lazygit
2023/08/06 23:04:00 Language not found: zh
23:04:00 ~/.c/lazygit 213ms $                                                                                               ✘ 1

23:05:19 ~/.c/lazygit 1ms $ js-yaml config.yml | jq '.gui.language'
"auto"
23:05:22 ~/.c/lazygit 128ms $ lazygit
当前目录不在 git 仓库中。是否在此目录创建一个新的 git 仓库？(y/n): ^C⏎
23:05:28 ~/.c/lazygit 4.45s $                                                                                             ✘ 130

[Lutra-Fs]

It seems that the language code 'zh-TW' should also be included.

[jesseduffield]

Thanks for making this