Fix typos (?) in tuples section

[ahjones]

I hope you'll find this helpful - I've started reading through the tutorial and I think that there are some small improvements that can be made in tuples.md.

I think there are some typos, which I've fixed:

• function -> function

• "the head is the bit first bit" -> "the head is the first bit"
• y -> y

[oubiwann]

Thanks!