doc: css: fix versions menu style

Versions menu dl > dt background color was not correct.

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 9154ee4..2e14bfa 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -899,7 +899,7 @@
     padding-left: 0 !important;
 }
 
-.rst-content dl:not(.docutils) dl dt, dl:not(.docutils,.rst-other-versions) dt {
+.rst-content dl:not(.docutils) dl:not(.rst-other-versions) dt {
     background: var(--admonition-note-background-color) !important;
     border-top: none !important;
     border-left: none !important;