fix: remove typo in duplicate key error

The message previously contained "at line", but
we don't have a way of knowing the line number so
there was nothing after that. This commit removes
the "at line" part of the message.

src/util.rs

@@ -172,7 +172,7 @@ where
 			while let Some((key, value)) = access.next_entry()? {
 				if map.contains_key(&key) {
 					return Err(serde::de::Error::custom(format!(
-						"duplicate key `{key}` at line"
+						"duplicate key `{key}`"
 					)));
 				}