GN Doc fix. Visibility default is public.

See Visibility::FillItemVisibility for code implementing default.

Review URL: https://codereview.chromium.org/608293002

Cr-Commit-Position: refs/heads/master@{#297237}

tools/gn/variables.cc

@@ -932,13 +932,13 @@ const char kVisibility_Help[] =
     "\n"
     "Examples\n"
     "\n"
-    "  Only targets in the current buildfile (\"private\", the default):\n"
+    "  Only targets in the current buildfile (\"private\"):\n"
     "    visibility = [ \":*\" ]\n"
     "\n"
     "  No targets (used for targets that should be leaf nodes):\n"
     "    visibility = []\n"
     "\n"
-    "  Any target (\"public\"):\n"
+    "  Any target (\"public\", the default):\n"
     "    visibility = [ \"*\" ]\n"
     "\n"
     "  All targets in the current directory and any subdirectory:\n"