rtc: Correct comment regarding rt_reduce_num_ref_buffers

Fixed a comment in av1_set_reference_structure_one_pass_rt()
regarding the assumptions used to enable the feature which
reduces the number of reference frame buffers.

Change-Id: I8c70fa0414cedaa19043bd529487abfa7ce0d33a
diff --git a/av1/encoder/ratectrl.c b/av1/encoder/ratectrl.c
index fc15581..eac1d3f 100644
--- a/av1/encoder/ratectrl.c
+++ b/av1/encoder/ratectrl.c
@@ -2683,7 +2683,7 @@
   }
   svc->gld_idx_1layer = gld_idx;
   // Set the flag to reduce the number of reference frame buffers used.
-  // This assumes that slot 7 is not refreshed by any reference frame.
+  // This assumes that slot 7 is never used.
   cpi->rt_reduce_num_ref_buffers = 1;
   cpi->rt_reduce_num_ref_buffers &= (svc->ref_idx[0] < 7);
   cpi->rt_reduce_num_ref_buffers &= (svc->ref_idx[1] < 7);