Doc: Set default character set for MySQL

The system's default (at least for MariaDB) is utf8mb4, and when
it is used - 255 chars for primary key is too long
(error: Specified key was too long; max key length is 767 bytes)

Change-Id: I1120f07079e6c9aa9a570d3da64aecd33216650a

Documentation/database-setup.txt

@@ -68,7 +68,7 @@ rights on it:
   mysql
 
   CREATE USER 'gerrit2'@'localhost' IDENTIFIED BY 'secret';
-  CREATE DATABASE reviewdb;
+  CREATE DATABASE reviewdb DEFAULT CHARACTER SET 'utf8';
   GRANT ALL ON reviewdb.* TO 'gerrit2'@'localhost';
   FLUSH PRIVILEGES;
 ----