doc: in the Docker example with custom plugins, replace ep_codepad -> ep_comments_page
doc: in the Docker example with custom plugins, replace ep_codepad -> ep_comments_page

Prefer promoting ep_comments_page, since it has a wider audience and is better
maintained.

ether/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite