Update upstream reference re 15.5-15.6 howto

[phillxnet]

When this document was written there was not specific 15.5 to 15.6 upstream references. There now is:

• https://en.opensuse.org/SDB:System_upgrade_to_Leap_15.6

It is poposed that this be used instead of the more generic and likely now misleading:

"Upstream reference: SDB:System upgrade" found in our existing howto.

I.e. switch the above note quote to:

"Upstream reference: SDB:System upgrade to Leap 15.6"