Sync flexible heredoc emulation with label fix

This was recently fixed in PHP via
php/php-src@3107088
so we should fix it here as well.

I'm not adding a dedicated test as it will fail until new PHP versions
are released. This is indirectly tested through the php-src tester.

lib/PhpParser/Lexer/Emulative.php

@@ -22,7 +22,7 @@ class Emulative extends Lexer
     const FLEXIBLE_DOC_STRING_REGEX = <<<'REGEX'
 /<<<[ \t]*(['"]?)([a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*)\1\r?\n
 (?:.*\r?\n)*?
-(?<indentation>\h*)\2(?![a-zA-Z_\x80-\xff])(?<separator>(?:;?[\r\n])?)/x
+(?<indentation>\h*)\2(?![a-zA-Z0-9_\x80-\xff])(?<separator>(?:;?[\r\n])?)/x
 REGEX;
 
     /** @var mixed[] Patches used to reverse changes introduced in the code */