Added ".opus" as recognized extension in the docs.

Author: BorisCarvajal

docs/src/refman/acodec.txt

@@ -13,7 +13,7 @@ This function registers all the known audio file type handlers for
 [al_load_sample], [al_save_sample], [al_load_audio_stream], etc.
 
 Depending on what libraries are available, the full set of recognised
-extensions is: .wav, .flac, .ogg, .it, .mod, .s3m, .xm, .voc.
+extensions is: .wav, .flac, .ogg, .opus, .it, .mod, .s3m, .xm, .voc.
 
 *Limitations:*