[librsvg: 15/21] contributing.rst: Fix markup

[Marge Bot <marge-bot src gnome org>]

commit 27889c781183c1233dc648e1d37998a1ac9aee2e
Author: Federico Mena Quintero <federico gnome org>
Date:   Wed Aug 17 17:47:53 2022 -0500

    contributing.rst: Fix markup
    
    Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/729>

 devel-docs/contributing.rst | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/devel-docs/contributing.rst b/devel-docs/contributing.rst
index 24e4d432b..d6c56231e 100644
--- a/devel-docs/contributing.rst
+++ b/devel-docs/contributing.rst
@@ -24,7 +24,8 @@ elsewhere, instead of hard-coding an actual value in every place it is
 used.  For example, define a ``button-color`` variable instead of
 hard-coding ``#aabbcc`` everywhere.
 
-::
+.. code-block:: css
+
    /* Define two variables */
    :root {
      --main-color: #06c;
@@ -55,7 +56,8 @@ machinery for CSS.
 
 One particularly interesting bit in Servo's implementation is that it guards against exponential expansion 
of malicious variables.
 
-::
+.. code-block:: css
+
    :root {
      --prop1: lol;
      --prop2: var(--prop1) var(--prop1);