python: Fix import in plugin example

When a plugin is being loaded from Python source, the binding will be
available under

    import libyosys

That is unfortunately different from how a self-standing Python program
would import the Yosys interface, which is

    from pyosys import libyosys

Until that is made consistent, at least fix the example to have it
working as is.

examples/python-api/pass.py

@@ -1,6 +1,6 @@
 #!/usr/bin/python3
 
-from pyosys import libyosys as ys
+import libyosys as ys
 
 import matplotlib.pyplot as plt
 import numpy as np