Add info how to assign drive numbers to 1st and 2nd internal drive via BASIC

[woepaul]

Describe where we can find the problematic topic

@gardners suggested I create an issue for this doc change request.
See https://discord.com/channels/719326990221574164/720355877701550131/1399344560039002174

Describe the solution you'd like

Add something like the following to the USING EXTERNAL IEC DISK DRIVES chapter:

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Alternatively to using the Freezer menu, the user can also change the unit number assignment of the internal drives via the BASIC POKE command:

To set the unit number of the 1st internal drive use:

POKE $10113, <new unit number>

To set the unit number of the 2nd internal drive use:

POKE $10114, <new unit number>

For example, if a user has an external 1571 drive configured to #9 and wants to use it:

POKE $10114, 11

can be used to switch the 2nd internal drive away from using #9 (to using #11), to make the external drive accessible with unit number 9.