docs: Add --cask option to brew command to suggest (#5432)

## What
- Add the `--cask` flag to the Homebrew update command for Codex.

## Why
- `brew upgrade codex` alone does not update the cask, so users were not
getting the right upgrade instructions.

## How
- Update `UpdateAction::BrewUpgrade` in `codex-rs/tui/src/updates.rs` to
use `upgrade --cask codex`.

## Testing
- [x] cargo test -p codex-tui

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-rs/tui/src/updates.rs

@@ -175,7 +175,7 @@ impl UpdateAction {
         match self {
             UpdateAction::NpmGlobalLatest => ("npm", &["install", "-g", "@openai/codex@latest"]),
             UpdateAction::BunGlobalLatest => ("bun", &["install", "-g", "@openai/codex@latest"]),
-            UpdateAction::BrewUpgrade => ("brew", &["upgrade", "codex"]),
+            UpdateAction::BrewUpgrade => ("brew", &["upgrade", "--cask", "codex"]),
         }
     }