Hide global option in `uv generate-shell-completion` (#6170)
Resolve #6152
## Summary
## Test Plan
Execution result of `cargo run generate-shell-completion --help`
```bash
Generate shell completion
Usage: uv generate-shell-completion <SHELL>
Arguments:
<SHELL> The shell to generate the completion script for [possible values: bash, elvish, fish, nushell, powershell, zsh]
```
Execution result of `cargo run help generate-shell-completion`
```bash
Generate shell completion
Usage: uv generate-shell-completion <SHELL>
Arguments:
<SHELL>
The shell to generate the completion script for
[possible values: bash, elvish, fish, nushell, powershell, zsh]
```
