rtc: Correct comment for ROI

Change-Id: Icd5f505681d283df5b7e1d032a82274bba0fc089

aom/aomcx.h

diff --git a/aom/aomcx.h b/aom/aomcx.h
index 708cd06..a0b0fe4 100644
--- a/aom/aomcx.h
+++ b/aom/aomcx.h
@@ -1640,7 +1640,7 @@
 typedef struct aom_roi_map {
   /*! If ROI is enabled. */
   uint8_t enabled;
-  /*! An id between 0 and 7 for each 8x8 region within a frame. */
+  /*! An id between 0 and 7 for each 4x4 region within a frame. */
   unsigned char *roi_map;
   unsigned int rows;               /**< Number of rows. */
   unsigned int cols;               /**< Number of columns. */