docs (scrypt): use ScryptParams::recommended() in usage example

[cbzehner]

This PR changes the example in scrypt to generate the params using ScryptParams::recommended(), per discussion in #49.

Confirmed it's working by running doc tests.

❯ cargo test --doc
    Finished test [unoptimized + debuginfo] target(s) in 0.02s
   Doc-tests scrypt

running 1 test
test src/lib.rs -  (line 13) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[newpavlov]

Thank you!