Carbon zephyr example location update (#959)

* iot: carbon: Fixed path to shell example

The module got moved [1] and renamed [2] upstream.

[1] zephyrproject-rtos/zephyr@52448f6
[2] zephyrproject-rtos/zephyr@fa7f13e

* iot: carbon: Fix markdown headings

A space was missing here, breaking the generation of proper headings.

iot/carbon/build/linux/build-shell.md

@@ -15,21 +15,21 @@ These instructions will show how to build a sample Zephyr application using a Li
 
 ***
 
-####**Step 0**: Download and setup Zephyr
+#### **Step 0**: Download and setup Zephyr
 - Download and setup the Zephyr development environment from https://www.zephyrproject.org. You can find more information about installing the Zephyr SDK at https://www.zephyrproject.org/doc/getting_started/getting_started.html
 
 ```shell
 $ git clone https://gerrit.zephyrproject.org/r/zephyr
 ```
 
-####**Step 1**: Build the Zephyr shell application
+#### **Step 1**: Build the Zephyr shell application
 
 **Build the sample shell application as follows:**
 
 ```shell
 $ cd zephyr
 $ source zephyr-env.sh
-$ cd samples/subsys/shell
+$ cd samples/subsys/shell/shell_module/
 $ mkdir outdir; cd outdir
 $ cmake -DBOARD=96b_carbon ..
 $ make