MDEV-4968 Old advices in mysql-log-rotate script

* Change the comments in mysql-log-rotate.sh to refer to mysqld, not mysqld_safe
  as that's what most distros are using.
* Change err-log to log-error as err-log is no longer valid.
* Convert tab to space for consistency.

support-files/mysql-log-rotate.sh

 # This logname can be set in /etc/my.cnf
-# by setting the variable "err-log"
-# in the [safe_mysqld] section as follows:
+# by setting the variable "log-error"
+# in the [mysqld] section as follows:
 #
-# [safe_mysqld]
-# err-log=@localstatedir@/mysqld.log
+# [mysqld]
+# log-error=@localstatedir@/mysqld.log
 #
 # If the root user has a password you have to create a
 # /root/.my.cnf configuration file with the following