zsh-completions: fix code style

Signed-off-by: Otavio Salvador <[e-mail]>

pkgs/shells/zsh/zsh-completions/default.nix

···
-{ lib, stdenv, fetchFromGitHub}:
+{ lib, stdenv, fetchFromGitHub }:
 
 stdenv.mkDerivation rec {
   pname = "zsh-completions";
···
   };
 
   strictDeps = true;
-  installPhase= ''
+  installPhase = ''
     install -D --target-directory=$out/share/zsh/site-functions src/*
   '';