fix integration test by updating a translation

Author: php-coder

src/main/resources/ru/mystamps/i18n/ValidationMessages.properties

@@ -30,7 +30,7 @@ ru.mystamps.web.service.dto.DownloadResult.INVALID_REDIRECT = URL must not redir
 ru.mystamps.web.service.dto.DownloadResult.INVALID_FILE_TYPE = Invalid file type
 ru.mystamps.web.service.dto.DownloadResult.INVALID_FILE_SIZE = Invalid file size
 ru.mystamps.web.service.dto.DownloadResult.FILE_NOT_FOUND = File not found
-ru.mystamps.web.service.dto.DownloadResult.UNEXPECTED_ERROR = Could not download a file
+ru.mystamps.web.service.dto.DownloadResult.UNEXPECTED_ERROR = Could not download file
 
 value.too-short = Value is less than allowable minimum of {min} characters
 value.too-long = Value is greater than allowable maximum of {max} characters