doc: Update Linaro release mentioned on index page

doc: Update Linaro release mentioned on index page

The version of the Linaro release that is used for testing was
updated in 35010bb8 and the user guide was updated with the
correct version, however the version is also mentioned on the
index page and that was missed. Update the index page with the
new version.

We can come back and de-duplicate this content later, to ease
future maintenance.

Change-Id: I3fe83d7a1c59ab8d3ce2b18bcc23e16c93f7af97
Signed-off-by: Paul Beesley <paul.beesley@arm.com>