Fix help message for --global-motion-method

The default global motion estimation method was changed to disflow a
while ago, but I forgot to change the default listed in the help text.
Fix that now.

Change-Id: Ib282ae563527835bf933a5d044fb0b4862e4cb5b
diff --git a/av1/arg_defs.c b/av1/arg_defs.c
index 8cab501..73dca03 100644
--- a/av1/arg_defs.c
+++ b/av1/arg_defs.c
@@ -702,7 +702,7 @@
               "given lambda to minimize the rdcost."),
   .global_motion_method = ARG_DEF_ENUM(NULL, "global-motion-method", 1,
                                        "Global motion search method "
-                                       "(default: feature-match)",
+                                       "(default: disflow):",
                                        global_motion_method_enum),
 #endif  // CONFIG_AV1_ENCODER
 };