fix(/collection/{slug}): the query for countries statistic was failing on PostgreSQL.

The error was:
org.springframework.jdbc.BadSqlGrammarException: PreparedStatementCallback; bad SQL grammar [SELECT
COALESCE(CASE WHEN 'ru' = ? THEN COALESCE(c.name_ru, c.name) ELSE c.name END, 'Unknown') AS name ,
SUM(s.quantity) AS counter FROM collections_series cs JOIN series s ON s.id = cs.series_id LEFT
JOIN countries c ON c.id = s.country_id WHERE cs.collection_id = ? GROUP BY s.country_id];
nested exception is org.postgresql.util.PSQLException: ERROR: column "c.name_ru" must appear in
the GROUP BY clause or be used in an aggregate function

Part of #1140

Author: php-coder

src/main/resources/sql/country_dao_queries.properties

@@ -65,7 +65,7 @@ country.count_stamps_by_countries = \
 LEFT JOIN countries c \
        ON c.id = s.country_id \
     WHERE cs.collection_id = :collection_id \
- GROUP BY s.country_id
+ GROUP BY c.id
 
 country.find_ids_by_names = \
    SELECT c.id \