Correct the heredoc regexp.

[antoineB]

To support heredoc in this style:
<<< 'HEREDOC'
instead of
<<<'HEREDOC'

[cjohansson]

If you want to use the same RegEx as PHP does you can see it here: https://github.com/cjohansson/emacs-phps-mode/blob/master/phps-mode-lexer.el#L1546

There can be tabs prefixed as well, the label is not any characters and there needs to be a newline following the label

[zonuexe]

@antoineB Thanks!