docs(changelog): update install command to @openai/codex@<version> (#2073)

# What

Updates the install command in the changelog template (`cliff.toml`)
from
```
npm install -g codex@version
```
to
```
npm install -g @openai/codex@<version>
```

# Why

The current command is incorrect, it tries installs the old “codex”
static site generator rather than the OpenAI Codex CLI.

# How

Edited only the header string in `cliff.toml` to point to
@openai/codex@<version>. No changelog regeneration or other files
touched.


Fixes #2059

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

cliff.toml

@@ -4,7 +4,7 @@
 header = """
 # Changelog
 
-You can install any of these versions: `npm install -g codex@version`
+You can install any of these versions: `npm install -g @openai/codex@<version>`
 """
 
 body = """