Correct asset path in generator.

src/generator.lua

@@ -715,7 +715,10 @@ M.generate_site = function()
     dir.makepath(M.asset_dir)
 
     -- Copy static assets.
-    dir.clonetree("templates/assets", M.asset_dir, dir.copyfile)
+    assert(dir.clonetree(
+        datafile.path("templates/assets"),
+        M.asset_dir, dir.copyfile)
+    )
 
     -- Clear local search index keys.
     idx_keys = {