docs: add missing leading slashes in help patterns

[ghost]

Fixes: #5857

[codecov-commenter]

Codecov Report

Merging #5869 (0e42f1d) into master (35d2587) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5869      +/-   ##
==========================================
+ Coverage   83.44%   83.49%   +0.04%     
==========================================
  Files          38       38              
  Lines       10353    10353              
  Branches     1710     1710              
==========================================
+ Hits         8639     8644       +5     
+ Misses       1208     1205       -3     
+ Partials      506      504       -2     

Impacted Files	Coverage Δ
src/borg/archiver.py	80.70% <ø> (+0.09%)	⬆️
src/borg/repository.py	83.92% <0.00%> (-0.19%)	⬇️
src/borg/archive.py	82.55% <0.00%> (+0.31%)	⬆️

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[ThomasWaldmann]

thanks!