python-tornado: fix license name

Upstream source (https://github.com/facebook/tornado

) says this is released
under version 2.0 of the Apache license which we have in the common licenses
directory as 'Apache-2.0'.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>