doc: scripts: redirects: fix redirect for arduino_uno_r4_minima

The directory name in boards has been changed with the addition of
support for Arduino UNO R4 WiFi, so add a redirect from
boards/arduino/uno_r4_minima to boards/arduino/uno_r4.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>