False info in README.md

This table says "implies `--enable-manager`", but I spent a good hour reinstalling, pulling, restoring backups and more trying to get `--enable-manager-legacy-ui` to work.

But in the end the solution was that I needed to add **both** flags for legacy ui to show.

`--enable-manager-legacy-ui` alone doesn't even make manager settings visible in options.

| Flag | Description |
|------|-------------|
| `--enable-manager` | Enable ComfyUI-Manager |
| `--enable-manager-legacy-ui` | Use the legacy manager UI instead of the new UI (implies `--enable-manager`) |
| `--disable-manager-ui` | Disable the manager UI and endpoints while keeping background features like security checks and scheduled installation completion (requires `--enable-manager`) |

Update the flag table in README.md, focusing on the descriptions for --enable-manager and --enable-manager-legacy-ui. Verify that the documentation states both flags are needed for the legacy UI and that the legacy flag alone does not enable manager settings visibility.

Written by the indexing model from the issue text.