Fix warning in unit test

Author: roberthdevries

pandas/tests/io/excel/test_openpyxl.py

@@ -114,7 +114,7 @@ def test_to_excel_with_openpyxl_engine(ext, tmpdir):
     df2 = DataFrame({"B": np.linspace(1, 20, 10)})
     df = pd.concat([df1, df2], axis=1)
     styled = df.style.applymap(
-        lambda val: "color: %s" % "red" if val < 0 else "black"
+        lambda val: "color: %s" % ("red" if val < 0 else "black")
     ).highlight_max()
 
     filename = tmpdir / "styled.xlsx"