[gnome-builder] doc: fix missing language in code-block directive

[Sébastien Lafargue <slafargue src gnome org>]

commit 8b49038d3db4597df49616d208079b3245df26d8
Author: Sebastien Lafargue <slafargue gnome org>
Date:   Sat Nov 17 12:15:49 2018 +0100

    doc: fix missing language in code-block directive
    
    Due to this error, the block was invisible in the generated doc:
    
    diagnostics.rst:46: WARNING: Error in "code-block" directive

 doc/help/plugins/editor/diagnostics.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/doc/help/plugins/editor/diagnostics.rst b/doc/help/plugins/editor/diagnostics.rst
index 70ebc6e2f..7adfc1974 100644
--- a/doc/help/plugins/editor/diagnostics.rst
+++ b/doc/help/plugins/editor/diagnostics.rst
@@ -43,7 +43,7 @@ You also need to register the plugin as a diagnostic provider in the ``.plugin``
 
 For example, a C diagnostic plugin will have a plugin file that look similar to this:
 
-.. code-block::
+.. code-block:: none
 
     # my_plugin.plugin