More user friendly vpxenc help strings.

Change-Id: I257fb32673c47848258dc7e6e22147d0beb49f86

vpxenc.c

diff --git a/vpxenc.c b/vpxenc.c
index cb91d94..d14625b 100644
--- a/vpxenc.c
+++ b/vpxenc.c
@@ -351,11 +351,11 @@
     NULL, "frame-parallel", 1, "Enable frame parallel decodability features");
 static const arg_def_t aq_mode = ARG_DEF(
     NULL, "aq-mode", 1,
-    "Adaptive q mode (0: off (by default), 1: variance 2: complexity, "
+    "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,
-    "Enable frame periodic boost (0: off (by default), 1: on)");
+    "Enable frame periodic boost (0: off (default), 1: on)");
 
 static const arg_def_t *vp9_args[] = {
   &cpu_used, &auto_altref, &noise_sens, &sharpness, &static_thresh,