Fix issue probably-meant-fstring found at https://codereview.doctor (#46855)

Author: code-review-doctor

pandas/tests/groupby/transform/test_transform.py

@@ -1043,7 +1043,7 @@ def demean_rename(x):
         if isinstance(x, Series):
             return result
 
-        result = result.rename(columns={c: "{c}_demeaned" for c in result.columns})
+        result = result.rename(columns={c: f"{c}_demeaned" for c in result.columns})
 
         return result