Fixes #113.

Summary

• update the README lock example to pass the Callable into Atomic's constructor
• call Atomic#call() with no arguments
• fix the anonymous Callable<Void> method signature to return Void

Notes

This is intentionally README-only: the production API and tests already use the new Atomic<>(callable, lock) shape. I did not change the separate Atomic Javadoc example mentioned in #115.

Verification

• git diff --check