doc: Set correct syntax highlighting style

Several code blocks do not specify a language for syntax
highlighting. This results in Sphinx using a default highlighter
which is Python.

This patch adds the correct language to each code block that doesn't
already specify it.

Change-Id: Icce1949aabfdc11a334a42d49edf55fa673cddc3
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/components/romlib-design.rst b/docs/components/romlib-design.rst
index 9f62b7c..ab39723 100644
--- a/docs/components/romlib-design.rst
+++ b/docs/components/romlib-design.rst
@@ -106,7 +106,7 @@
 In the below example the usage of ROMLIB together with mbed TLS is demonstrated
 to showcase the benefits of library at ROM - it's not mandatory.
 
-::
+.. code:: shell
 
     make PLAT=fvp                                                   \
     MBEDTLS_DIR=</path/to/mbedtls/>                                 \