Fix for Library/Phalcon/Utils/Slug.php: locale is en_US.UTF-8

The correct locale spelling is UTF-8, not UTF8

```bash
$ locale
LANG=uk_UA.UTF-8
LANGUAGE=
LC_CTYPE="uk_UA.UTF-8"
LC_NUMERIC="uk_UA.UTF-8"
LC_TIME="uk_UA.UTF-8"
LC_COLLATE="uk_UA.UTF-8"
LC_MONETARY="uk_UA.UTF-8"
LC_MESSAGES="uk_UA.UTF-8"
LC_PAPER="uk_UA.UTF-8"
LC_NAME="uk_UA.UTF-8"
LC_ADDRESS="uk_UA.UTF-8"
LC_TELEPHONE="uk_UA.UTF-8"
LC_MEASUREMENT="uk_UA.UTF-8"
LC_IDENTIFICATION="uk_UA.UTF-8"
LC_ALL=
```