Update `OFFICIAL_MESSAGE` on `src/main/java/com/infobip/ApiException.java` to use "please" and "submit"

[plinkr]

Updates the error message to use "please" and "submit" instead of "contact" and "open an issue". This makes the message more polite and consistent with the rest of the library's tone. Changes on src/main/java/com/infobip/ApiException.java:

    private static final String OFFICIAL_MESSAGE =
            "If you are having trouble interpreting the error, please contact " + SUPPORT_EMAIL
                    + " or submit an issue on our GitHub repo. " + "Include all details you find relevant. "
                    + "Thank you for using our library!";