Fix namespace usage in test_merge.py

Author: lexzlei

pandas/tests/reshape/merge/test_merge.py

@@ -3152,8 +3152,8 @@ def test_merge_pyarrow_datetime_duplicates():
 
 
 def test_merge_right_on_and_right_index():
-    df1 = pd.DataFrame({"col": [1, 2, 3]})
-    df2 = pd.DataFrame({"col": [2, 3, 4]})
+    df1 = DataFrame({"col": [1, 2, 3]})
+    df2 = DataFrame({"col": [2, 3, 4]})
 
     with pytest.raises(pd.errors.MergeError):
         df1.merge(df2, left_on="col", right_on="col", right_index=True)