Update deprecated method usage in encryption example

[funsiyuan]

Hello, this is just a small PR for update README example code.

Goals

Update the code example in the documentation to replace the deprecated withUnsafeMutableBytes method with a more modern and safe approach.

Expected results

The documentation should now include an example that does not use any deprecated methods, ensuring it remains current and free of warnings.

Actual results

The documentation previously included an example that used the deprecated withUnsafeMutableBytes method, which could lead to warnings or potential issues in future Swift versions.

Steps to reproduce

1. Open the Realm Swift documentation.
2. Locate the example code that generates a random encryption key.
3. Observe the use of the deprecated withUnsafeMutableBytes method.

Code sample that highlights the issue

// Generate a random encryption key
var key = Data(count: 64)
_ = key.withUnsafeMutableBytes { bytes in
    SecRandomCopyBytes(kSecRandomDefault, 64, bytes)
}

// Add the encryption key to the config and open the realm
let config = Realm.Configuration(encryptionKey: key)
let realm = try Realm(configuration: config)

// Use the Realm as normal
let dogs = realm.objects(Dog.self).filter("name contains 'Fido'")

[funsiyuan]

@tgoyne Hi,
this is a small PR. Hope someone review this. Thx.