Doc: Updated the external link to the OpenJDK web site

The earlier link was pointing to the OpenJDK download page
for linux distributions only. The new one links to the
OpenJDK generic page which talks about the JDK versions for
other platforms as well.

Task-number: QTBUG-30173
Change-Id: I5fef543e79c2e5c9b3eb86b2e9ad8169b80a4ab0
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>