Typo in trainer.py doc (#597)

Missing " character

Author: raacee

src/setfit/trainer.py

@@ -284,7 +284,7 @@ class Trainer(ColumnMappingMixin):
         column_mapping (`Dict[str, str]`, *optional*):
             A mapping from the column names in the dataset to the column names expected by the model.
             The expected format is a dictionary with the following format:
-            `{"text_column_name": "text", "label_column_name: "label"}`.
+            `{"text_column_name": "text", "label_column_name": "label"}`.
     """
 
     def __init__(