docs: fix typo in README (essential -> essentially)

[biggzfs-sys]

Summary

• Fixes a small grammatical typo in README.md: "essential separate copies" should read "essentially separate copies" (adverb, not adjective).

Test plan

• Docs-only change, no functional code touched.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}