Remove ivfenc usage message leading underscores

An earlier automatic transform changed eg '\nOptions' to '\n_options'
which is incorrect in these printfs. Fix these.

Change-Id: I7e0f37931ef82b79fadddd7058ce0df5572e2ca1
diff --git a/ivfdec.c b/ivfdec.c
index 3919d6b..8065e70 100644
--- a/ivfdec.c
+++ b/ivfdec.c
@@ -113,7 +113,7 @@
             "Options:\n", exec_name);
     arg_show_usage(stderr, all_args);
 #if CONFIG_VP8_DECODER
-    fprintf(stderr, "\nvp8 Postprocessing Options:\n");
+    fprintf(stderr, "\nVP8 Postprocessing Options:\n");
     arg_show_usage(stderr, vp8_pp_args);
 #endif
     fprintf(stderr, "\nIncluded decoders:\n\n");