doc: fix figure style CSS selector

The CSS classes used by figures in docutils has changed in recent
versions. Adjust CSS so that figures are centered again by default.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/doc/_static/css/custom.css b/doc/_static/css/custom.css
index 4ecea90..45e4887 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -864,7 +864,7 @@
     column-width: 18em;
 }
 
-.figure {
+.rst-content div.figure, .rst-content figure {
     text-align: center;
 }