Fix a warning in test_plot3d_matrix_color
Silence the following warning:
```
[WARNING]: 0.5c not a valid number and may not be decoded properly.
```
@@ -498,7 +498,7 @@ def test_plot3d_matrix_color(data, region):
p="225/30",
R="/".join(map(str, region)),
J="X5i",
- S="c0.5cc",
+ S="c0.5c",
C="rainbow",
i="0,1,2,2",
B=["a", "za"],
