[Nucleo L432KC] Fix pin mapping

D7/D8 was inverted, moreover add a comment to specify
they are not connected by default.
SB6 and SB8 need to be closed to connect them to the CN3
connector.

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

Author: fpistm

variants/NUCLEO_L432KC/variant.cpp

@@ -31,8 +31,8 @@ const PinName digitalPin[] = {
   PB_7,  //D4
   PB_6,  //D5
   PB_1,  //D6
-  PC_14, //D7
-  PC_15, //D8
+  PC15,  //D7 - By default, SB8 open PF1/PC15 not connected to D7
+  PC14,  //D8 - By default, SB6 open PF0/PC14 not connected to D8
   PA_8,  //D9
   PA_11, //D10
   PB_5,  //D11

variants/NUCLEO_L432KC/variant.h

@@ -40,8 +40,8 @@ extern const PinName digitalPin[];
 #define PB7  4
 #define PB6  5
 #define PB1  6
-#define PC14 7
-#define PC15 8
+#define PC15 7  // By default, SB8 open PF1/PC15 not connected to D7
+#define PC14 8  // By default, SB6 open PF0/PC14 not connected to D8
 #define PA8  9
 #define PA11 10
 #define PB5  11