Fixed the link to api reference in the manual.

The problem is well described in issue #61

> http://projects.haskell.org/diagrams/doc/manual.html#other-resources
> says "The API reference documentation for all the diagrams packages is
> intended to be high-quality and up-to-date, and is available from the
> diagrams website.". It links to
> http://projects.haskell.org/diagrams/doc/index.html, but the latter
> does not exist.
Merge pull request #63 from zudov/reference_link_fix

Fixed the link to api reference in the manual.

Closes #61.