Fix missing option '-e' in zpool online usage

[Low-power]

Description

This fixes the usage message of zpool online command.

Motivation and Context

How Has This Been Tested?

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)
• 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.
• Change has been approved by a ZFS on Linux member.

[codecov]

Codecov Report

Merging #7655 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7655      +/-   ##
==========================================
- Coverage   77.94%   77.83%   -0.12%     
==========================================
  Files         366      366              
  Lines      110618   110618              
==========================================
- Hits        86217    86095     -122     
- Misses      24401    24523     +122

Flag	Coverage Δ
#kernel	78.4% <ø> (+0.03%)	⬆️
#user	66.69% <100%> (-0.13%)	⬇️

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