Fixed typo in itertools documentation

[Casper-Smet]

Fixed typo in itertools documentation

As I was perusing the documentation for itertools.product, I noticed a small typo in the last sentence:

Accordingly, it only useful with finite inputs.

This pull request simply adds the word "is" in order to make this sentence grammatically correct:

Accordingly, it is only useful with finite inputs.

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I also added my name to the misc/ACKS file, per the contribution guidelines.

[the-knights-who-say-ni]

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[Casper-Smet]

I have now signed the CLA and signed up to b.p.o., my sincere apologies for not doing this earlier.

[fochoao]

It is useful for finite inputs.