Update basics.rst

The basic example rendering the given appstruct does not work, appstruct must be a dictionary and not a list otherwise colander will throw the following exception:

Invalid: {'': u'"[{\'age\': 20, \'name\': \'keith\'}, {\'age\': 23, \'name\': \'fred\'}]" is not a mapping type: Does not implement dict-like functionality.'}

I am using colander v1.0b1.