Updated deduplication section in zfsconcepts.7 for clarity

[PuvvadaBhaskar]

Motivation and Context #2829

The Deduplication section in zfsconcepts.7 contained complex wording that could be made more clear for new users.
This change improves readability and clarity while maintaining the original meaning.

Description

De

Reworded and simplified explanations in the Deduplication section.

Improved grammar and sentence flow.

No functional, performance, or behavior changes.

Documentation-only update.

How Has This Been Tested?

This is a documentation-only change — no code modifications were made.
Verified formatting using .man syntax highlighting in VS Code and confirmed consistent structure with other sections.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [ *] Documentation (a change to man pages or other documentation)

Checklist:

• [* ] My code follows the OpenZFS code style requirements.
• [* ] I have updated the documentation accordingly.
• [* ] I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• [ *] All commit messages are properly formatted and contain Signed-off-by.