Clarify how different program types are used

The Sway book says the following:
> A Sway program itself has a type: it is either a contract, a predicate, a script, or a library. The first three of these things are all deployable to the blockchain. A library is simply a project designed for code reuse and is never directly deployed to the chain.

It's a bit misleading to say that scripts and predicates are "deployable". In fact, only contracts are. We should clarify the language here.

Locate the quoted passage in the Sway book and read the surrounding explanation of contracts, predicates, scripts, and libraries. Clarify that only contracts are deployable, while preserving the intended distinction for reusable libraries, then run the documentation checks to confirm the book builds cleanly.

Written by the indexing model from the issue text.