Fix a typo in in opus_custom.h.

Signed-off-by: Ralph Giles <giles@thaumas.net>
diff --git a/include/opus_custom.h b/include/opus_custom.h
index 41f36bf..2227be0 100644
--- a/include/opus_custom.h
+++ b/include/opus_custom.h
@@ -178,7 +178,7 @@
 ) OPUS_ARG_NONNULL(1);
 
 
-/** Destroys a an encoder state.
+/** Destroys an encoder state.
   * @param[in] st <tt>OpusCustomEncoder*</tt>: State to be freed.
   */
 OPUS_CUSTOM_EXPORT void opus_custom_encoder_destroy(OpusCustomEncoder *st);
@@ -286,7 +286,7 @@
     int *error
 ) OPUS_ARG_NONNULL(1);
 
-/** Destroys a an decoder state.
+/** Destroys a decoder state.
   * @param[in] st <tt>OpusCustomDecoder*</tt>: State to be freed.
   */
 OPUS_CUSTOM_EXPORT void opus_custom_decoder_destroy(OpusCustomDecoder *st);