fix(commands/mr/approve): use a more precise description for --sha

[maxice8]

Description

Use a more accurate description for the --sha flag

How Has This Been Tested?

bin/glab mr approve --help

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[codecov]

Codecov Report

Merging #543 (d5b8a16) into trunk (631e7d8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk     #543   +/-   ##
=======================================
  Coverage   60.66%   60.66%           
=======================================
  Files          86       86           
  Lines        5880     5880           
=======================================
  Hits         3567     3567           
  Misses       1979     1979           
  Partials      334      334           

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