aom/aomcx.h: fix AOME_SET_CQ_LEVEL comment

rc_end_usage should be set to AOM_CQ, not g_usage.

BUG=aomedia:2354

Change-Id: Ibb7e496408066b0c6152c87db768403c5ffd420e
diff --git a/aom/aomcx.h b/aom/aomcx.h
index da7498f..e8af1df 100644
--- a/aom/aomcx.h
+++ b/aom/aomcx.h
@@ -220,8 +220,8 @@
 
   /*!\brief Codec control function to set constrained quality level.
    *
-   * \attention For this value to be used aom_codec_enc_cfg_t::g_usage must be
-   *            set to #AOM_CQ.
+   * \attention For this value to be used aom_codec_enc_cfg_t::rc_end_usage
+   *            must be set to #AOM_CQ.
    * \note Valid range: 0..63
    */
   AOME_SET_CQ_LEVEL,