modespec: Correct timings for Robot 8 B/W

The timings as originally given in this file equate to a scan period of
60.16ms, which is a little over.  QSSTV seems to be much happer if we
give it 59.9ms scan lines.

Decoding https://www.sigidwiki.com/images/3/37/Robot_8_BW.mp3 with the
original timings leads to a slanted image.  By adjusting the timings, we
can correct this without needing further processing.

modespec.c

@@ -198,8 +198,8 @@ _ModeSpec ModeSpec[] = {
     .SyncTime     = 7e-3,
     .PorchTime    = 0e-3,
     .SeptrTime    = 0e-3,
-    .PixelTime    = 0.188e-3,
-    .LineTime     = 67e-3,
+    .PixelTime    = 0.1871875e-3,
+    .LineTime     = 66.9e-3,
     .ImgWidth     = 320,
     .NumLines     = 120,
     .LineHeight   = 2,