Fix DatabaseCommand::execute() return type hint
Fix DatabaseCommand::execute() return type hint

mongodb/mongo-php-library,mongodb/mongo-php-library,derickr/mongo-php-library,jmikola/mongo-php-library-prototype,mongodb-labs/mongo-php-library-prototype,jmikola/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,derickr/mongo-php-library-prototype,10gen-labs/mongo-php-library-prototype,derickr/mongo-php-library,jmikola/mongo-php-library