Fix docs issue

Author: znicholls

src/pandas_openscm/db/backends.py

@@ -66,7 +66,7 @@ def guess_backend(self, data_file_name: str) -> OpenSCMDBDataBackend:
 
         Parameters
         ----------
-        index_file_name
+        data_file_name
             Name of the data file from which to guess the backend
 
         Returns