Make install command in documentation compatible with fish shell

[doubledup]

With the fish shell support added earlier this year, Nix supports installations in fish shell out of the box. The installation instructions still use process substitution syntax that's specific to other shells (eg. bash & zsh), so users installing Nix from fish shell must translate the command before the installation will work.

This PR makes the install commands in the installation instructions compatible with fish shell.

Running the current install command in fish produces

$ sh <(curl -L https://nixos.org/nix/install)
...
fish: Invalid redirection target: 
sh <(curl -L https://nixos.org/nix/install)
   ^

Running the command with the correct fish syntax starts the installer (also tested with the --daemon flag passed to the installer)

$ curl -L https://nixos.org/nix/install | sh
...
Switching to the Multi-user Installer
Welcome to the Multi-User Nix Installation
...

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[doubledup]

I have read the CLA Document and I hereby sign the CLA

[fricklerhandwerk]

As commented on your other PR we could just as well use

curl -L https://nixos.org/nix/install | sh

[doubledup]

@fricklerhandwerk That's much better, thanks! I've updated the PR and the description.