Update SlowSoftI2CMaster link. MightyBoard digipots I2C bus has no resistors.

Marlin/digipot_mcp4018.cpp

@@ -26,7 +26,7 @@
 
 #include "Stream.h"
 #include "utility/twi.h"
-#include <SlowSoftI2CMaster.h>  //https://github.com/felias-fogg/SlowSoftI2CMaster
+#include <SlowSoftI2CMaster.h>  //https://github.com/stawel/SlowSoftI2CMaster
 
 // Settings for the I2C based DIGIPOT (MCP4018) based on WT150