[ticket/12750] Center installation page copyright text.

PHPBB3-12750
Merge pull request #2646 from prototech/ticket/12750

[ticket/12750] Center installation page copyright text.

* prototech/ticket/12750:
  [ticket/12750] Center installation page copyright text.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12750] Center installation page copyright text.