Admin manual: Fix typo in MySQL/MariaDB properties

- Change dashes to underscores in the property names. Verified the names
  via the following documentation pages:
  - MySQL: https://dev.mysql.com/doc/refman/8.0/en/server-option-variable-reference.html
  - MariaDB: https://mariadb.com/kb/en/server-system-variables/

Signed-off-by: Simon Westersund <swestersund@users.noreply.github.com>

admin_manual/configuration_database/linux_database_configuration.rst

@@ -70,8 +70,8 @@ Your :file:`/etc/mysql/my.cnf` could look like this:
   query_cache_size = 64M
   tmp_table_size= 64M
   max_heap_table_size= 64M
-  slow-query-log = 1
-  slow-query-log-file = /var/log/mysql/slow.log
+  slow_query_log = 1
+  slow_query_log_file = /var/log/mysql/slow.log
   long_query_time = 1
 
   [client-server]
@@ -82,8 +82,8 @@ Your :file:`/etc/mysql/my.cnf` could look like this:
   default-character-set = utf8mb4
 
   [mysqld]
-  character-set-server = utf8mb4
-  collation-server = utf8mb4_general_ci
+  character_set_server = utf8mb4
+  collation_server = utf8mb4_general_ci
   transaction_isolation = READ-COMMITTED
   binlog_format = ROW
   innodb_large_prefix=on