Fix what I think is a typo in WebVTT test data

Without the CSS tweak the additional test assertion fails.

PiperOrigin-RevId: 288698323

library/core/src/test/assets/webvtt/with_css_complex_selectors

 WEBVTT
 
 STYLE
-::cue(\n#id ){text-decoration:underline;}
+::cue(#id ){text-decoration:underline;}
 
 STYLE
 ::cue(#id.class1.class2 ){ color: violet;}

library/core/src/test/java/com/google/android/exoplayer2/text/webvtt/WebvttDecoderTest.java

@@ -326,6 +326,7 @@ public class WebvttDecoderTest {
   public void testWithComplexCssSelectors() throws Exception {
     WebvttSubtitle subtitle = getSubtitleForTestAsset(WITH_CSS_COMPLEX_SELECTORS);
     Spanned firstCueText = getUniqueSpanTextAt(subtitle, /* timeUs= */ 0);
+    assertThat(firstCueText).hasUnderlineSpanBetween(0, firstCueText.length());
     assertThat(firstCueText)
         .hasForegroundColorSpanBetween(
             "This should be underlined and ".length(), firstCueText.length())