Update CODEOWNERS

[oddlittlebird]

What this PR does / why we need it: Add our new tech writer to the Loki docs as a codeowner. @achatterjee-grafana

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[codecov-commenter]

Codecov Report

Merging #2579 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2579      +/-   ##
==========================================
+ Coverage   63.03%   63.05%   +0.02%     
==========================================
  Files         169      169              
  Lines       15005    15005              
==========================================
+ Hits         9458     9461       +3     
+ Misses       4794     4784      -10     
- Partials      753      760       +7     

Impacted Files	Coverage Δ
pkg/promtail/targets/file/tailer.go	70.83% <0.00%> (-4.17%)	⬇️
pkg/promtail/targets/file/filetarget.go	66.85% <0.00%> (-2.29%)	⬇️
pkg/querier/queryrange/downstreamer.go	95.87% <0.00%> (-2.07%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️
pkg/promtail/positions/positions.go	59.64% <0.00%> (+13.15%)	⬆️

[slim-bean]

LGTM!