[ticket/12637] Correctly escape the file header in coding-guidelines.html.

PHPBB3-12637
[ticket/12637] Fixup all incorrect uses of file header.

PHPBB3-12637
Merge pull request #2524 from bantu/ticket/12637

[ticket/12637] Correct file header in coding-guidelines

* bantu/ticket/12637:
  [ticket/12637] Fixup all incorrect uses of file header.
  [ticket/12637] Correctly escape the file header in coding-guidelines.html.