fix typo: Not a binary operator: =

[eraac]

What this PR does / why we need it: Fix a typo in common.libsonnet

Special notes for your reviewer:

$ tk show environments/loki
evaluating jsonnet: RUNTIME ERROR: [...]/loki/vendor/loki/common.libsonnet:10:66-67 Not a binary operator: =
	[...]/loki/vendor/loki/loki.libsonnet:4:2-27	$
	main.jsonnet:2:14-42	thunk <loki> from <$>
	main.jsonnet:4:1-5	$
	During evaluation

[sh0rez]

My bad, that shouldn't have been merged in the first place.

LGTM for me 🚀

/cc @slim-bean @rfratto

[rfratto]

LGTM as well, thanks for fixing this!

[codecov-io]

Codecov Report

Merging #1776 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1776      +/-   ##
==========================================
+ Coverage   64.34%   64.38%   +0.04%     
==========================================
  Files         121      121              
  Lines        9086     9086              
==========================================
+ Hits         5846     5850       +4     
+ Misses       2836     2834       -2     
+ Partials      404      402       -2

Impacted Files	Coverage Δ
pkg/ingester/transfer.go	66.42% <0%> (+1.42%)	⬆️
pkg/promtail/targets/tailer.go	78.4% <0%> (+2.27%)	⬆️