chore: fix Nebula image link in README

Trivial fix to README to fix image link for Nebula extension.

Signed-off-by: Joseph Glanville <jglanville@groq.com>
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

README.md

@@ -100,7 +100,7 @@ cosign verify --certificate-identity-regexp '@siderolabs\.com$' --certificate-oi
 | Name                                | Image                                                                                                 | Description                                               | Version Format     |
 | ----------------------------------- | ----------------------------------------------------------------------------------------------------- | --------------------------------------------------------- | ------------------ |
 | [cloudflared](network/cloudflared/) | [ghcr.io/siderolabs/cloudflared](https://github.com/siderolabs/extensions/pkgs/container/cloudflared) | [Cloudflared](https://github.com/cloudflare/cloudflared/) | `upstream version` |
-| [nebula](network/nebula/)           | [ghcr.io/siderolabs/cloudflared](https://github.com/siderolabs/extensions/pkgs/container/nebula)      | [Nebula](https://github.com/slackhq/nebula)               | `upstream version` |
+| [nebula](network/nebula/)           | [ghcr.io/siderolabs/nebula](https://github.com/siderolabs/extensions/pkgs/container/nebula)           | [Nebula](https://github.com/slackhq/nebula)               | `upstream version` |
 | [lldpd](network/lldpd/)             | [ghcr.io/siderolabs/lldpd](https://github.com/siderolabs/extensions/pkgs/container/lldpd)             | [LLDP](https://github.com/lldpd/lldpd)                    | `upstream version` |
 | [tailscale](network/tailscale/)     | [ghcr.io/siderolabs/tailscale](https://github.com/siderolabs/extensions/pkgs/container/tailscale)     | [Tailscale](https://tailscale.com)                        | `upstream version` |