style: Fix comma position after heredoc call argument (#383)

The previous version (with the comma on the following line) dates back
to PHP < 7.3.0 when the heredoc terminator had to be the only thing on
the line.

php-cs-fixer 3.86.0 updates the symfony ruleset to require that the
comma be on the same line as the heredoc terminator (which we already do
in most places), meaning this line is now reported as a violation.

Author: acoulton

tests/Keywords/KeywordsDumperTest.php

@@ -220,8 +220,8 @@ public function testExtendedVersionDumper(): void
                 Примеры:
                   | agent1 | agent2 |
                   | D      | M      |
-            GHERKIN
-            , <<<'GHERKIN'
+            GHERKIN,
+            <<<'GHERKIN'
             # language: ru
             Фича: Internal operations
               In order to stay secret