board/raspberrypi: readme: add a missing word

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/raspberrypi/readme.txt

@@ -23,7 +23,7 @@ Configure Buildroot
 -------------------
 
 The raspberrypi_defconfig configuration is a minimal configuration with
-all that is required to bring the Raspberry Pi. You should base your
+all that is required to bring the Raspberry Pi up. You should base your
 work on this defconfig:
 
   $ make raspberrypi_defconfig