lavfi: math typo in interlace filter

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavfilter/vf_interlace.c

@@ -150,7 +150,7 @@ static void copy_picture_field(AVFrame *src_frame, AVFrame *dst_frame,
                     srcp_below = srcp; // there is no line below
                 for (i = 0; i < linesize; i++) {
                     // this calculation is an integer representation of
-                    // '0.5 * current + 0.25 * above + 0.25 + below'
+                    // '0.5 * current + 0.25 * above + 0.25 * below'
                     // '1 +' is for rounding.
                     dstp[i] = (1 + srcp[i] + srcp[i] + srcp_above[i] + srcp_below[i]) >> 2;
                 }