docs: fix syntax error in plugin installation example
docs: fix syntax error in plugin installation example

Partially addresses issue #4368.

stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,jsha/letsencrypt,stweil/letsencrypt