ibm.1440: Increase gap3 from 84 bytes to 108 bytes

This better matches the disks formatted by IBM PCs, and also matches
the recommended "Format GAP3" in Table 4-1 of
"PC8477B (SuperFDC) Advanced Floppy Disk Controller"
<https://www.ardent-tool.com/datasheets/NS_PC8477B.pdf>

Fixes #568

Author: keirf

src/greaseweazle/data/diskdefs_ibm.cfg

@@ -84,7 +84,7 @@ disk 1440
     tracks * ibm.mfm
         secs = 18
         bps = 512
-        gap3 = 84
+        gap3 = 108
         rate = 500
     end
 end