update php error log directory

previously this guide informed the user to set their php error log as ```php.log``` in ```/var/log/```:
```error_log = /var/log/php.log ```

after which it tells the user to create ```/var/log/php``` and set the permissions on that directory:
```mkdir /var/log/php```
```chown www-data /var/log/php```

the fix is to set the error_log to a file in ```/var/log/php/``` and then create and chown that directory.