Fix comment: size_align is in pixels, not bytes.

size_align is applied to the image width and height, which is in pixels.

Change-Id: Ib63f0589208b13c9254567d84454d42be0796b6c
diff --git a/aom/aom_image.h b/aom/aom_image.h
index bc0c6a6..5f615d7 100644
--- a/aom/aom_image.h
+++ b/aom/aom_image.h
@@ -259,7 +259,7 @@
  * \param[in]    d_h        Height of the image
  * \param[in]    align      Alignment, in bytes, of the image buffer and
  *                          each row in the image (stride).
- * \param[in]    size_align Alignment, in bytes, of the image width and height.
+ * \param[in]    size_align Alignment, in pixels, of the image width and height.
  * \param[in]    border     A border that is padded on four sides of the image.
  *
  * \return Returns a pointer to the initialized image descriptor. If the img