Backport PR #10775: Correct Demo namespace in documentation

The documentation used the old namespace for the Demo class.

IPython/lib/demo.py

@@ -106,7 +106,7 b' the execution.'
 This is probably best explained with the simple example file below.  You can
 copy this into a file named ex_demo.py, and try running it via::
 
-    from IPython.demo import Demo
+    from IPython.lib.demo import Demo
     d = Demo('ex_demo.py')
     d()