[docs] ntuple forward reference

[Julien Palard]

Hi Finch, thanks for reporting!

>  In the Python Tutorial, the  term "tuple" is used a number of times long  before it is defined.

I opened a PR to add a forward reference:
https://github.com/python/cpython/pull/13499

For the other missing forward reference of tuple being mentionned in List Comprehensions before being introduced I think we should move 5.3 Tuples before 5.1.3 List Comprehensions, what do you think?

-- 
Julien Palard
https://mdk.fr