Improve indexOf sample to showcase return value handling

Remove sample reference from indexOf(Char) overload: both overloads are on
the same page, and showing the sample for indexOf(String) under indexOf(Char)
is redundant and misleading.

libraries/stdlib/src/kotlin/text/Strings.kt

@@ -984,7 +984,6 @@ public fun CharSequence.lastIndexOfAny(strings: Collection<String>, startIndex:
  *
  * @param ignoreCase `true` to ignore character case when matching a character. By default `false`.
  * @return An index of the first occurrence of [char] or -1 if none is found.
- * @sample samples.text.Strings.indexOf
  */
 public fun CharSequence.indexOf(char: Char, startIndex: Int = 0, ignoreCase: Boolean = false): Int {
     return if (ignoreCase || this !is String)