Correct a typo where the wrong effect ID was used for vibrato waveform

E4x is the correct ID, not E3x, for MOD and XM.
Additionally, updated the coding style for MOD to be more consistent with the
pre-existing code, namely by not using hex for the effect IDs being checked.

src/engine/fileOps/xm.cpp

@@ -1147,7 +1147,7 @@ bool DivEngine::loadXM(unsigned char* file, size_t len) {
               case 0xe: // special...
                 // TODO: implement the rest
                 switch (effectVal>>4) {
-                  case 0x3: // vibrato waveform
+                  case 0x4: // vibrato waveform
                     switch (effectVal&3) {
                       case 0x0: // sine
                         p->data[j][effectCol[k]++]=0xe3;