[BLACK_F407] Fix BTN_K1 definition

Fix #387 thanks @BennehBoy

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

variants/BLACK_F407VE/variant.h

@@ -148,7 +148,7 @@ extern const PinName digitalPin[];
 // On-board user button
 #define BTN_K_UP                PA0
 #define BTN_K0                  PE4
-#define BTN_K1                  PE5
+#define BTN_K1                  PE3
 #define USER_BTN                BTN_K0
 
 // Below SPI and I2C definitions already done in the core