Tweak comment in PgsDecoder to match the structure of the data

The Palette Definition Segment is documented here, you can is it goes:
luminance (y), red (cr), blue (cb), alpha

http://blog.thescorpius.com/index.php/2017/07/15/presentation-graphic-stream-sup-files-bluray-subtitle-format/

(I think it's important to be really precise here, because YCbCr is a
very common format, so this ordering is slightly unusual:
https://en.wikipedia.org/wiki/YCbCr)

PiperOrigin-RevId: 350747808

library/core/src/main/java/com/google/android/exoplayer2/text/pgs/PgsDecoder.java

@@ -134,7 +134,7 @@ public final class PgsDecoder extends SimpleSubtitleDecoder {
 
     private void parsePaletteSection(ParsableByteArray buffer, int sectionLength) {
       if ((sectionLength % 5) != 2) {
-        // Section must be two bytes followed by a whole number of (index, y, cb, cr, a) entries.
+        // Section must be two bytes then a whole number of (index, Y, Cr, Cb, alpha) entries.
         return;
       }
       buffer.skipBytes(2);