Misleading comment in pg_upgrade.c

Hello,

While reading pg_upgrade code to restore the objects on the new
cluster, I noticed that 5b570d771b8 didn't adjust the database name in
the comments explaining the requirements for an extra "--clean" for
template1 and postgres databases.  While it's true that both databases
will already exist, I found it confusing to mention both names when
only one is processed for each code path.