flux-fsck: fix duplicate option character

[chu11]

Problem: In flux-fsck both the --rootref and --repair options use the same short option character 'r'.

Use the character 'R' for --repair instead of 'r'. Update documentation accordingly.

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Ooops!!!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.66%. Comparing base (ddaaf3c) to head (62f6920).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7169      +/-   ##
==========================================
+ Coverage   83.64%   83.66%   +0.02%     
==========================================
  Files         551      551              
  Lines       91891    91891              
==========================================
+ Hits        76859    76882      +23     
+ Misses      15032    15009      -23     

Files with missing lines	Coverage Δ
src/cmd/builtin/fsck.c	83.80% <ø> (ø)

... and 10 files with indirect coverage changes

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