Documentation for adding with claude cli is wrong

The command at https://github.com/github/github-mcp-server/blob/main/docs/installation-guides/install-claude.md#with-docker is correct

But the command in README.md is wrong

```
claude mcp update github -e GITHUB_PERSONAL_ACCESS_TOKEN=$GITHUB_PAT
```

The `update` command does not exist. Looks like someone updated the install guide but forgot to fix README.md