Updated the haskell.nix imports to overlay style

Some of the examples in the documentation were still passing an empty
set to the haskell.nix imports but this is no longer valid because the
import now results in a set rather than a function.

Author: toonn

docs/user-guide/cabal-projects.md

@@ -81,7 +81,7 @@ instantiate a package set.
 # default.nix
 let
   # Import the Haskell.nix library,
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz);
 
   # Instantiate a package set using the generated file.
   pkgSet = haskell.mkCabalProjectPkgSet {

docs/user-guide/development.md

@@ -108,7 +108,7 @@ selects packages from the larger package set.
 ```nix
 # shell.nix
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz);
 in
   haskell.haskellPackages.ghcWithPackages (ps: with ps;
     [ lens conduit conduit-extra ])
@@ -125,7 +125,7 @@ project.
 
 ```nix
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz);
 in
   haskell.snapshots."lts-13.18".alex.components.exes.alex
 ```

docs/user-guide/stack-projects.md

@@ -43,7 +43,7 @@ instantiate a package set.
 # default.nix
 let
   # Import the Haskell.nix library,
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz);
 
   # Instantiate a package set using the generated file.
   pkgSet = haskell.mkStackPkgSet {