tests.haskell.documentationTarball: switch to random

We only need *a* package, to test the "documentationTarball" function,
not specifically vector. I guess we should use a package that is rarely
broken itself... for which a *dependency* of vector should be a good
choice. I took a random one... "random"!

pkgs/test/haskell/documentationTarball/default.nix

@@ -1,7 +1,7 @@
 { pkgs, haskellPackages }:
 
 let
-  drv = haskellPackages.vector;
+  drv = haskellPackages.random;
   docs = pkgs.haskell.lib.compose.documentationTarball drv;
 
 in
@@ -15,10 +15,10 @@ pkgs.runCommand "test haskell.lib.compose.documentationTarball"
     tar xvzf "${docs}/${drv.name}-docs.tar.gz"
 
     # Check for Haddock html
-    find "${drv.name}-docs" | grep -q "Data-Vector.html"
+    find "${drv.name}-docs" | grep -q "System-Random.html"
 
     # Check for source html
-    find "${drv.name}-docs" | grep -q  "src/Data.Vector.html"
+    find "${drv.name}-docs" | grep -q  "src/System.Random.html"
 
     touch "$out"
   ''