docs: AE3 battery shield SD card baudrate 20 MHz -> 12 MHz

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: kwagyeman

docs/openmvcam/shields/ae3-battery-shield.rst

@@ -94,7 +94,7 @@ The microSD socket is exposed on the AE3's SPI bus through
     spi = machine.SPI(0)
     cs  = machine.Pin("P3", machine.Pin.OUT, value=1)
 
-    sd  = sdcard.SDCard(spi, cs, baudrate=20_000_000)
+    sd  = sdcard.SDCard(spi, cs, baudrate=12_000_000)
     os.mount(os.VfsFat(sd), "/sdcard")
     print(os.listdir("/sdcard"))