Fix docker driver doc

[petuhovskiy]

Had some trouble today understanding why loki driver stopped recognizing log-opts after upgrade. It was, until I figured out that grafana/loki-docker-driver:master image is actually 1 year old, and I should use latest instead.

This PR fixes Upgrading section by replacing incorrect tag in snippet.

By the way, consider the option of updating/deleting master tag in docker hub registry. It can eliminate this kind of problem.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #2420 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2420      +/-   ##
==========================================
- Coverage   61.58%   61.58%   -0.01%     
==========================================
  Files         160      160              
  Lines       13590    13590              
==========================================
- Hits         8370     8369       -1     
  Misses       4597     4597              
- Partials      623      624       +1     

Impacted Files	Coverage Δ
pkg/promtail/targets/file/tailer.go	76.13% <0.00%> (-2.28%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️
pkg/promtail/targets/file/filetarget.go	70.48% <0.00%> (+1.80%)	⬆️

[owen-d]

Thanks, you're totally right on all accounts. We keep master-<sha> tags currently if you're looking for a specific build, too.