upipe_alsa_source doxy comment refers to another pipe

[JDarnley]

upipe/lib/upipe-alsa/upipe_alsa_source.c

Lines 493 to 494 in d1bdd59

	/** @internal @This sets the content of an avcodec option. It only take effect
	* after the next call to @ref upipe_avcdec_set_url.

Probably an error from an old copy-paste that was never changed.