[Infra] Use - instead of * when moving changelog entries around

The merge back action uses the code in `Changelog.kt` to ensure
unreleased entries in the CHANGELOG.md files are moved to the
corresponding section after being released.

Since we have started formatting our markdown files, we rely on `-` as
a list marker, rather than `*`, which was the old format.

This change updates `Changelog.kt` to use the right character.

Author: rlazo

plugins/src/main/java/com/google/firebase/gradle/plugins/Changelog.kt

@@ -326,7 +326,7 @@ data class ReleaseContent(val subtext: String, val changes: List<Change>) {
  */
 data class Change(val type: ChangeType, val message: String) {
 
-  override fun toString(): String = "* [$type] $message"
+  override fun toString(): String = "- [$type] $message"
 
   companion object {
     /**