[docs] [issue29089] dictionary keys described incorrectly in tutorial

[INADA Naoki]

INADA Naoki added the comment:

> If precision is the problem, could it be reworded to be more vague?

"can be any immutable" seems vague enough to me.  It doesn't say "cannot be any mutable type.".

>  Users are likely to assume documentation from any official source is the final word.

But tutorial should use less words as possible.  This section is before introducing "class" statement.  Only strings, numbers, list and set are introduced before section.

I feel assuming early part of tutorial as final word bad practice.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29089>
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