Fix CSS instructions in CONTRIBUTING.md

[silverwind]

Fixed the task naming, which is actually generate-stylesheets. Also my editor seems to have picked up a whole bunch of trailing whitespaces. I hope it's ok to keep that in this commit.

[codecov-io]

Codecov Report

Merging #3053 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3053      +/-   ##
==========================================
+ Coverage   33.34%   33.37%   +0.02%     
==========================================
  Files         270      270              
  Lines       39566    39566              
==========================================
+ Hits        13195    13205      +10     
+ Misses      24484    24471      -13     
- Partials     1887     1890       +3

Impacted Files	Coverage Δ
models/issue_indexer.go	64.93% <0%> (-2.6%)	⬇️
models/error.go	32.2% <0%> (-0.62%)	⬇️
models/issue.go	44.7% <0%> (-0.21%)	⬇️
modules/indexer/repo.go	63.47% <0%> (+2.6%)	⬆️
models/repo_indexer.go	51.98% <0%> (+6.43%)	⬆️

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[techknowlogick]

LGTM

[sapk]

LGTM