Update documentation for uniques (This PR renames classes and instances to...

Update documentation for uniques (This PR renames classes and instances to collections and items in the Uniques pallet in order to follow the commonly accepted NFTs terminology.) (#13322)

* Update documentation for uniques

The documentation was outdated after merge of #11389
Using the widely spread term collections and item instead
of the previous class and instance.

* Update README.md

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Co-authored-by: Jegor Sidorenko <[email protected]>
Co-authored-by: Jegor Sidorenko <[email protected]>