docs: fix codeblock in getting_started.rst

Issue was uncovered by sphinx-build:
docs/user/getting_started.rst:100: WARNING: Definition list ends without a blank line; unexpected unindent.

docs/user/getting_started.rst

@@ -93,7 +93,7 @@ In case of errors read the messages carefully and try to fix the stated issues
 ``ar71xx-generic`` is the most common target and will generate images for most of the supported hardware.
 To see a complete list of supported targets, call ``make`` without setting ``GLUON_TARGET``.
 
-To build all targets use a loop like this:
+To build all targets use a loop like this::
 
     for TARGET in $(make list-targets); do
       make GLUON_TARGET=$TARGET