Fix a typo in class_audiostreamsynchronized.rst

[brevven]

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

Context:

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

[mhilbrunner]

Thanks for contributing!

Note that at the top of the file, it says:

.. DO NOT EDIT THIS FILE!!!
.. Generated automatically from Godot engine sources.
.. Generator: https://github.com/godotengine/godot/tree/master/doc/tools/make_rst.py.
.. XML source: https://github.com/godotengine/godot/tree/master/modules/interactive_music/doc_classes/AudioStreamSynchronized.xml.

As this file is autogenerated from the file in the engine repository, all changes made here will be lost. Please open a PR in the Godot Engine repository to edit the XML file mentioned above to make these changes persist. :)