Update epdif.cpp

Correct spelling of include file (SPI.h) for all OS (now only compile for Windows, which is not care about upper/lower case)

Arduino/epd1in54_V2/epdif.cpp

@@ -26,7 +26,7 @@
  */
 
 #include "epdif.h"
-#include <spi.h>
+#include <SPI.h>
 
 EpdIf::EpdIf() {
 };
@@ -61,4 +61,4 @@ int EpdIf::IfInit(void) {
     SPI.begin();
     SPI.beginTransaction(SPISettings(2000000, MSBFIRST, SPI_MODE0));
     return 0;
-}
+}