fix error display

tokenizers/src/models/wordpiece/mod.rs

@@ -15,7 +15,7 @@ impl std::error::Error for Error {}
 impl fmt::Display for Error {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         match self {
-            MissingUnkToken => write!(
+            Error::MissingUnkToken => write!(
                 fmt,
                 "WordPiece error: Missing [UNK] token from the vocabulary"
             ),
@@ -158,3 +158,13 @@ impl Model for WordPiece {
         self.vocab_r.get(&id).map(|token| token.clone())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_error_display() {
+        assert!(format!("{}", Error::MissingUnkToken).contains("Missing [UNK] token"));
+    }
+}