Renaming vpxenc option from frame_boost to frame-boost.

We use dashes, no underscores. 

Change-Id: I423eee946d10731856e15591eb6e9e4628e3ee6c
diff --git a/vpxenc.c b/vpxenc.c
index d8fc553..9fa2883 100644
--- a/vpxenc.c
+++ b/vpxenc.c
@@ -378,7 +378,7 @@
     "Adaptive quantization mode (0: off (default), 1: variance 2: complexity, "
     "3: cyclic refresh)");
 static const arg_def_t frame_periodic_boost = ARG_DEF(
-    NULL, "frame_boost", 1,
+    NULL, "frame-boost", 1,
     "Enable frame periodic boost (0: off (default), 1: on)");
 
 static const struct arg_enum_list tune_content_enum[] = {