vi: modify Northern Vietnamese voice for clearer sound

espeak-ng-data/lang/aav/vi

@@ -2,4 +2,15 @@ name Vietnamese (Northern)
 language vi
 
 words 1
-pitch 100 180
+pitch 80 160
+
+formant 1 87 82 76
+formant 2 94 56 42
+formant 3 93 52 130
+formant 4 110 76 65
+formant 5  102 45 20
+formant 6  40 50 50
+formant 7 60 50 60
+formant 8 100 50 40
+
+tone 900 125  1600 100 2400 80