h264: Fix a typo from the previous commit

f777504f64 changed a - in +

CC: libav-stable@libav.org

libavcodec/h264.c

@@ -3896,7 +3896,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             }
         }
     }
-    h->qp_thresh = 15 +
+    h->qp_thresh = 15 -
                    FFMIN(h->slice_alpha_c0_offset, h->slice_beta_offset) -
                    FFMAX3(0,
                           h->pps.chroma_qp_index_offset[0],