stm32f4-discovery: correct part number for alternatives

STM32F401 Discovery is labelled MB1115B:
http://www.st.com/stm32f401discovery

Author: fb

examples/stm32/f4/stm32f4-discovery/README.md

@@ -7,6 +7,6 @@ accelerometer.
 
 http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/PF252419
 
-The later stm32f4 discovery boards, for the f401, MB91115B and for the f429i,
+The later stm32f4 discovery boards, for the f401, MB1115B and for the f429i,
 MB1075B will not work as is with these demos, but they should only need minor
 tweaking for pin configurations.