RELEASE_NOTES: fix WebVTT decoder typo (-> encoder)

Found-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 065fa66872)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

RELEASE_NOTES

@@ -105,7 +105,7 @@
      Other interesting new features including hqx video filter, a pixel art
      scaling filter; a fixed-point AC-3 decoder contributed by Imagination
      Technologies; an On2 TrueMotion VP7 video decoder; an HTML5 WebVTT
-     subtitle decoder that allows creation of WebVTT from any text-based
+     subtitle encoder that allows creation of WebVTT from any text-based
      subtitles; and an 1-bit Direct Stream Digital audio decoder.
 
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