DOC/FIX: fix example in ctypeslib module documentation

Fix some inconsistencies in the example. The library is loaded as '_lib' not
'lib' in the rest of the example. Also, the function is called 'foo_func' not
'foo'
Merge pull request #2960 from esc/doc/fix/ctypeslib

DOC/FIX: fix example in ctypeslib module documentation