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Add @edolstra suggestion fixes.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libexpr/primops/fetchTree.cc

@@ -529,17 +529,16 @@ static void prim_fetchurl(EvalState & state, const PosIdx pos, Value * * args, V
 
 static RegisterPrimOp primop_fetchurl({
     .name = "__fetchurl",
-    .args = {"args"},
+    .args = {"arg"},
     .doc = R"(
-      If args is a URL, return the path of the downloaded file.
-      Otherwise, it can be an attribute with the following attributes
-      (all except url are optional):
+      Download the specified URL and return the path of the downloaded file.
+      `arg` can be either a string denoting the URL, or an attribute set with the following attributes:
 
       - `url`
 
         The URL of the file to download.
 
-      - `name` (default: `url without the protocol`)
+      - `name` (default: the last path component of the URL)
 
         A name for the file in the store. This can be useful if the URL has any
         characters that are invalid for the store.