Fix URL for matplotib intersphinx inventory

doc/conf.py

@@ -25,5 +25,5 @@ intersphinx_mapping = {
     "https://documen.tician.de/pytential/": None,
     "https://documen.tician.de/boxtree/": None,
     "https://docs.sympy.org/latest/": None,
-    "https://matplotlib.org/": None,
+    "https://matplotlib.org/stable/": None,
 }