Update usage

image_build/rustup.sh

@@ -21,14 +21,14 @@ RUSTUP_UPDATE_ROOT="https://static.rust-lang.org/rustup/dist"
 #XXX: If you change anything here, please make the same changes in setup_mode.rs
 usage() {
     cat 1>&2 <<EOF
-rustup-init 1.0.0 (408ed84 2017-02-11)
+rustup-init 1.0.0b (2018-12-16)
 The installer for rustup
 
 USAGE:
     rustup-init [FLAGS] [OPTIONS]
 
 FLAGS:
-    -d, --dir               Set install directory
+    -d, --dir <path>        Set install directory
     -v, --verbose           Enable verbose output
     -y                      Disable confirmation prompt.
         --no-modify-path    Don't configure the PATH environment variable