[pica-etherpad-db] Fix error in Dockerfile

pica-etherpad-db/Dockerfile

 FROM mysql:8
 
-COPY config/limit_binary_log.cnf /etc/mysql/conf.d/limit_binary_log.cnf
+COPY config/binary_log.cnf /etc/mysql/conf.d/binary_log.cnf
 
 # By default, COPY uses permissions `-rw-rw-rw-` for the file inside
 # the container. But mysql refuses to use config files that are world-