Fix typo in AudioStreamSynchronized.xml

Fix a typo ("being" -> "begin")

Context:

>The streams begin at exactly the same time when play is pressed

modules/interactive_music/doc_classes/AudioStreamSynchronized.xml

@@ -4,7 +4,7 @@
 		Stream that can be fitted with sub-streams, which will be played in-sync.
 	</brief_description>
 	<description>
-		This is a stream that can be fitted with sub-streams, which will be played in-sync. The streams being at exactly the same time when play is pressed, and will end when the last of them ends. If one of the sub-streams loops, then playback will continue.
+		This is a stream that can be fitted with sub-streams, which will be played in-sync. The streams begin at exactly the same time when play is pressed, and will end when the last of them ends. If one of the sub-streams loops, then playback will continue.
 	</description>
 	<tutorials>
 	</tutorials>