Fix instructions for updating Mbed

Author: relativityspace-jsmith

README.md

@@ -4,7 +4,7 @@ This project implements a basic hello world using Mbed OS Community Edition.  Us
 ## How to set up this project:
 
 1. Clone it to your machine.  Don't forget to use `--recursive` to clone the submodules: `git clone --recursive https://github.com/mbed-ce/mbed-ce-hello-world.git`
-2. You may want to update the mbed-os submodule to the latest version, with `cd mbed-os && git pull origin master`
+2. You may want to update the mbed-os submodule to the latest version, with `cd mbed-ce-hello-world/mbed-os && git fetch origin && git reset --hard origin/master`
 3. Set up the GNU ARM toolchain (and other programs) on your machine using [the toolchain setup guide](https://github.com/mbed-ce/mbed-os/wiki/Toolchain-Setup-Guide).
 4. Set up the CMake project for editing.  We have three ways to do this:
     - On the [command line](https://github.com/mbed-ce/mbed-os/wiki/Project-Setup:-Command-Line)