Fix doxy of fixed_mean_energy parameter in ff_amr_set_fixed_gain().

Originally committed as revision 23987 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/acelp_pitch_delay.h

@@ -221,7 +221,7 @@ int16_t ff_acelp_decode_gain_code(
  * Calculate fixed gain (part of section 6.1.3 of AMR spec)
  *
  * @param fixed_gain_factor gain correction factor
- * @param fixed_energy decoded algebraic codebook vector energy
+ * @param fixed_mean_energy mean decoded algebraic codebook vector energy
  * @param prediction_error vector of the quantified predictor errors of
  *        the four previous subframes. It is updated by this function.
  * @param energy_mean desired mean innovation energy