Fix sync_binlog name in mysql_variables example

The previous sync_binary_log is not a real MySQL variable.

database/mysql_variables

@@ -56,8 +56,8 @@ options:
             - unix socket to connect mysql server
 '''
 EXAMPLES = '''
-# Check for sync_binary_log setting
+# Check for sync_binlog setting
-- mysql_variables: variable=sync_binary_log
+- mysql_variables: variable=sync_binlog
 
 # Set read_only variable to 1
 - mysql_variables: variable=read_only value=1