update error message for missing '0x' prefix (#6869)

crates/cairo-lang-utils/src/bigint.rs

@@ -31,7 +31,9 @@ where
     match s.strip_prefix("0x") {
         Some(num_no_prefix) => BigUint::from_str_radix(num_no_prefix, 16)
             .map_err(|error| serde::de::Error::custom(format!("{error}"))),
-        None => Err(serde::de::Error::custom(format!("{s} does not start with `0x` is missing."))),
+        None => Err(serde::de::Error::custom(format!(
+            "{s} does not start with `0x`, which is missing."
+        ))),
     }
 }