docs: fix simple typo, undifined -> undefined

There is a small typo in cookbook/c03/p07_inf_nan.py.

Should read `undefined` rather than `undifined`.

Author: timgates42

Diff for: cookbook/c03/p07_inf_nan.py

@@ -16,7 +16,7 @@ def inf_nan():
     print(a * 10 == a)
     print(10 / a)
 
-    # undifined
+    # undefined
     print(a / a)
     print(a + b)