Change "Vpx" to "Avx"

Change-Id: I0d1e439b75eb749dd80a86474ac5f494478d0360
diff --git a/webmenc.cc b/webmenc.cc
index 50aa85d..a987bf3 100644
--- a/webmenc.cc
+++ b/webmenc.cc
@@ -26,7 +26,7 @@
                             const aom_codec_enc_cfg_t *cfg,
                             const struct aom_rational *fps,
                             stereo_format_t stereo_fmt, unsigned int fourcc,
-                            const struct VpxRational *par) {
+                            const struct AvxRational *par) {
   mkvmuxer::MkvWriter *const writer = new mkvmuxer::MkvWriter(glob->stream);
   mkvmuxer::Segment *const segment = new mkvmuxer::Segment();
   segment->Init(writer);