linux-doc: fixup fetch after commit 9e68ab7b83d
linux-doc: fixup fetch after commit 9e68ab7b83d

Let's stop using src.override.  I see no advantage,
and the switch of original src from `sha256 =` to `hash =`
was causing really confusing errors.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs