[ticket/12708] Correct the path to the LICENSE file in install_main.

PHPBB3-12708
[ticket/12708] Make the path absolute by using __DIR__.

PHPBB3-12708
Merge pull request #2594 from bantu/ticket/12708

[ticket/12708] Fix install_main still linking to docs/COPYING
Merge branch 'develop-ascraeus' into develop

# By Andreas Fischer (1) and Huynh Buu Tam (1)
# Via Andreas Fischer (1) and Dhruv Goel (1)
* develop-ascraeus:
  [ticket/12708] Make the path absolute by using __DIR__.
  [ticket/12708] Correct the path to the LICENSE file in install_main.