Docs: Fix broken markdown with unescaped character

[cez81]

The left bracket is not escaped in the sed command causing tag|release to go missing in the generated document.

The defaults provided here are best-effort (not built automatically). They are accurately recorded in app.ini.sample (s/master/<tag|release\>).

https://docs.gitea.io/en-us/config-cheat-sheet/

[codecov-io]

Codecov Report

Merging #3470 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3470      +/-   ##
==========================================
+ Coverage   35.67%   35.69%   +0.01%     
==========================================
  Files         281      281              
  Lines       40706    40706              
==========================================
+ Hits        14523    14528       +5     
+ Misses      24033    24029       -4     
+ Partials     2150     2149       -1

Impacted Files	Coverage Δ
models/repo_list.go	67.18% <0%> (+1.56%)	⬆️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

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[lunny]

LGTM