Fix malformed string formatting

src/Notebook_Processor/desired_state/cell_modifier.py

@@ -155,13 +155,13 @@ class CellModifier(BaseModel):
         if signature[sig_keys[0]].kind not in (inspect.Parameter.POSITIONAL_ONLY, inspect.Parameter.POSITIONAL_OR_KEYWORD):
             raise ValueError(
                 f"Offending function signature: Action {self.action.__name__} must have the first parameter 'cell' as a positional argument.",
-                f"Found: {sig_keys[0], signature[sig_keys[0]]}",
+                f"Found: {sig_keys[0]}, {signature[sig_keys[0]]}",
                 )
 
         if signature[sig_keys[-1]].kind != inspect.Parameter.VAR_KEYWORD:
             raise ValueError(
                 f"Offending function signature: Action {self.action.__name__} must accept keyword arguments.",
-                f"The last paramter should be '**kwargs', found: {sig_keys[-1], signature[sig_keys[-1]].kind}",
+                f"The last paramter should be '**kwargs', found: {sig_keys[-1]}, {signature[sig_keys[-1]].kind}",
                 )
 
         missing_annotations = [