docs: raspberrrypi: rpi_pico: Fix broken RP2040 datasheet link

The RP2040 datasheet link in the rpi_pico board documentation was broken.

It has been updated to point to the correct location.

This change ensures consistency and prevents confusion, while keeping the
useful reference to the official RP2040 documentation.

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>