Fixed a small typo in man/man1/raidz_test.1

[pewo]

This is a small typo fix, and my first PR in zfs

Motivation and Context

Correct english

Description

I changed the word bencmarking to benchmarking

How Has This Been Tested?

N/A N/A N/A N/A

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)
• Breaking change (fix or feature that would cause existing functionality to change)
• [x ] Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #8855 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8855      +/-   ##
==========================================
- Coverage   78.67%   78.56%   -0.11%     
==========================================
  Files         382      382              
  Lines      117771   117771              
==========================================
- Hits        92651    92522     -129     
- Misses      25120    25249     +129

Flag	Coverage Δ
#kernel	79.35% <ø> (+0.05%)	⬆️
#user	67% <ø> (-0.42%)	⬇️

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