README.md: update the correct Nix channel

Correct the conmon Nix channel information to fix search error "Wrong channel selected!".

Signed-off-by: Haiyue Wang <[email protected]>

README.md

@@ -91,7 +91,7 @@ installed, like [runc](https://github.com/opencontainers/runc) or
 
 It is possible to build a statically linked binary of conmon by using
 the officially provided
-[nix](https://nixos.org/nixos/packages.html?attr=conmon&channel=nixpkgs-unstable&query=conmon)
+[nix](https://nixos.org/nixos/packages.html?attr=conmon&channel=unstable&query=conmon)
 package and the derivation of it [within this repository](nix/). The
 builds are completely reproducible and will create a x86\_64/amd64
 stripped ELF binary for [glibc](https://www.gnu.org/software/libc).