doc: Fix MCUboot capitalization

The correct spelling is "MCUboot", so replace the instances that differ
from this.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/boot/zephyr/targets/arduino_101.h b/boot/zephyr/targets/arduino_101.h
index cecd9d4..2a97268 100644
--- a/boot/zephyr/targets/arduino_101.h
+++ b/boot/zephyr/targets/arduino_101.h
@@ -21,10 +21,10 @@
 
 /*
  * NOTE: This flash layout is a simple one for demonstration purposes.
- * It assumes we are building MCUBoot as a 3rd-stage loader, leaving the
+ * It assumes we are building MCUboot as a 3rd-stage loader, leaving the
  * stock bootloaders on the Arduino 101 untouched.
  *
- * In this configuration MCUBoot will live at 0x40010000
+ * In this configuration MCUboot will live at 0x40010000
  */
 
 #define FLASH_DRIVER_NAME		CONFIG_SOC_FLASH_QMSI_DEV_NAME