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/plat/qemu.rst b/docs/plat/qemu.rst
index 30ae97d..4ebe64b 100644
--- a/docs/plat/qemu.rst
+++ b/docs/plat/qemu.rst
@@ -33,13 +33,13 @@
 
 To build:
 
-::
+.. code:: shell
 
     make CROSS_COMPILE=aarch64-none-elf- PLAT=qemu
 
 To start (QEMU v2.6.0):
 
-::
+.. code:: shell
 
     qemu-system-aarch64 -nographic -machine virt,secure=on -cpu cortex-a57  \
         -kernel Image                           \