factorio: use real instead of sample in download help

Makes it easier for the user since they should no longer have to
manually replace things.

pkgs/by-name/fa/factorio/package.nix

@@ -40,7 +40,7 @@ let
 
   mods = args.mods or [ ];
 
-  helpMsg = ''
+  helpMsg = { dlName, storeName }: ''
 
     ===FETCH FAILED===
     Please ensure you have set the username and token with config.nix, or
@@ -65,9 +65,7 @@ let
     download the release through https://factorio.com/download , then add it to
     the store using e.g.:
 
-      releaseType=alpha
-      version=0.17.74
-      nix-prefetch-url file://\''$HOME/Downloads/factorio_\''${releaseType}_x64_\''${version}.tar.xz --name factorio_\''${releaseType}_x64-\''${version}.tar.xz
+      nix-prefetch-url file://\''$HOME/Downloads/${dlName} --name ${storeName}
 
     Note the ultimate "_" is replaced with "-" in the --name arg!
   '';
@@ -146,7 +144,10 @@ let
                   '';
               failureHook = ''
                 cat <<EOF
-                ${helpMsg}
+                ${helpMsg {
+                  dlName = if candidateHashFilenames != [ ] then builtins.head candidateHashFilenames else name;
+                  storeName = name;
+                }}
                 EOF
               '';
             })