Add special X11 setup for decay.py

Fix the write/append operations, such that the proper X11 setup is written to
decay.py

README.md

@@ -329,7 +329,7 @@ import matplotlib
 matplotlib.use('Agg')
 ```
 -->
-<!-- write decay.py -->
+<!-- append decay.py -->
 ```python
 import numpy as np
 import scipy.optimize  # provides fit routines