Support external class both with and without a space after the colon

Author: rlazo

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

@@ -191,7 +191,7 @@ abstract class MakeReleaseNotesTask : DefaultTask() {
      */
     private val LINK_REGEX =
       Regex(
-        "(?:GitHub )?(?:\\[|\\()#(\\d+)(?:\\]|\\))(?:\\(.+?\\))?(?:\\{: \\.external\\})?",
+        "(?:GitHub )?(?:\\[|\\()#(\\d+)(?:\\]|\\))(?:\\(.+?\\))?(?:\\{:\\s*\\.external\\})?",
         RegexOption.MULTILINE,
       )
   }