schema: correct slim-projection count in header comment

The transactions table keeps 9 of the 16 source fields; the dropped
list also includes Card type (equals Card number in all samples, per
readme). Header said 10 and omitted Card type.

Documentation/schema.sql

@@ -15,10 +15,10 @@
 --    cleartext on purpose: customers retrieve it in the portal, and
 --    the cards are only used on-site at the station, so the risk is
 --    accepted.
---  - transactions is a SLIM PROJECTION (10 of 16 source fields):
---    QualityName, Station, Terminal, Pump, Card report group number
---    and Control number are dropped. The files table (full byte
---    content) is the canonical archive; the ledger keeps only what
+--  - transactions is a SLIM PROJECTION (9 of 16 source fields):
+--    QualityName, Card type, Station, Terminal, Pump, Card report
+--    group number and Control number are dropped. The files table (full
+--    byte content) is the canonical archive; the ledger keeps only what
 --    application features query.
 --  - Customer business key is the register customer number, stored as
 --    a STRING in both customers.external_id and