[horde] Suggestion - small change to Horde6 install instructions

[Sebastian Arcus]

Hello and thank you for all the work put into making Horde 6 happen.

I've just spent a number of hours trying to get Horde 6 working after it 
kept on crashing with "502 Bad Gateway" as soon as I configured the 
database access.

Eventually I realised that I needed to populate the database with tables 
with "horde-db-migrate up" - before trying to navigate any other 
sections of the admin interface.

Maybe there could be a note to say "don't try anything else before 
populating the database tables in the section below" - or something like 
this - to save other slightly clueless people like me from generating 
noise on this list :)