fix aliases in system-variables.md (#4045)#4046

Merged

ran-huang merged 2 commits intopingcap:release-4.0from ti-srebot:release-4.0-ae4e5e700865

Jul 24, 2020

+1-1

Commits on Jul 24, 2020

cherry pick pingcap#4045 to release-4.0

ran-huang
authored and
ti-srebot
committed
Update system-variables.md
ran-huang
authored