de: fix the HAPPY vowel for the mb-de5-en and mb-de6-en voices.

phsource/mbrola/de6

@@ -40,7 +40,7 @@
 0  **  NULL  0  R
 0  n^  NULL  40 n j
 
-// phonemes for English words
+// phonemes for English words (see docs/languages/gmw/en.md)
 0  @L  NULL  40 @ l
 0  m-  NULL  20 @ m
 0  n-  NULL  20 @ n
@@ -48,7 +48,11 @@
 0  @5  NULL  0  @
 0  a#  NULL  0  @
 0  I2  NULL  0  I
-0  i   NULL  0  i:
+// --------- HAPPY vowel:
+// * `i:` -- this is too long and gets clipped for the short HAPPY vowel, and de5 is missing `i:-w`.
+// * `i`  -- de5 and de6 are missing `l-i` (-ly), `R-i` (-ry), etc.
+// * `I`  -- works best (NOTE: This sounds like 'i', not like 'I'.)
+0  i   NULL  0  I
 0  aa  NULL  0  a
 0  V   NULL  0  a:
 0  0   NULL  0  a: