Small fix to dump_config

Author: tskisner

src/toast/config/cli.py

@@ -81,7 +81,7 @@ def load_config(file, input=None, comm=None):
         return ret
 
 
-def dump_config(file, conf, format="toml", comm=None):
+def dump_config(file, conf, format=None, comm=None):
     """Dump a config file to a supported format.
 
     Writes the configuration to a file in the specified format.  If `format` is