Missing reference in section 4.6 to chapter on classes

[9b1bb1e8-a4cb-4920-a80c-37ac8a1a6ceb]

BPO	6204
Nosy	@birkenfeld
Files	controlflow.rst.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = 'https://github.com/birkenfeld' closed_at = created_at = labels = ['docs'] title = 'Missing reference in section 4.6 to chapter on classes' updated_at = user = 'https://bugs.python.org/cofi' ``` bugs.python.org fields: ```python activity = actor = 'georg.brandl' assignee = 'georg.brandl' closed = True closed_date = closer = 'georg.brandl' components = ['Documentation'] creation = creator = 'cofi' dependencies = [] files = ['14192'] hgrepos = [] issue_num = 6204 keywords = ['patch'] message_count = 2.0 messages = ['88939', '89010'] nosy_count = 2.0 nosy_names = ['georg.brandl', 'cofi'] pr_nums = [] priority = 'normal' resolution = 'accepted' stage = None status = 'closed' superseder = None type = None url = 'https://bugs.python.org/issue6204' versions = ['Python 3.1'] ```

[9b1bb1e8-a4cb-4920-a80c-37ac8a1a6ceb]

In section 4.6 there is described that classes will be explained later on. I think a real reference would be more appropriate. See attached patch.

[birkenfeld]

Thanks, applied in r73258!