Fix a typo

Change-Id: I12de2dd5e5f375551804166188d76a9ad8067b41

vp9/encoder/vp9_rdopt.c

diff --git a/vp9/encoder/vp9_rdopt.c b/vp9/encoder/vp9_rdopt.c
index bcd8f01..9402a4a 100644
--- a/vp9/encoder/vp9_rdopt.c
+++ b/vp9/encoder/vp9_rdopt.c
@@ -3035,7 +3035,7 @@
   for (ref_frame = LAST_FRAME; ref_frame <= ALTREF_FRAME; ++ref_frame) {
     if (!(cpi->ref_frame_flags & flag_list[ref_frame])) {
       // Skip checking missing references in both single and compound reference
-      // modes. Note that a mode will be skipped iff both reference frames
+      // modes. Note that a mode will be skipped if both reference frames
       // are masked out.
       ref_frame_skip_mask[0] |= (1 << ref_frame);
       ref_frame_skip_mask[1] |= SECOND_REF_FRAME_MASK;