doc: update syntax highlighting color for accessibility

Previous contrast with background was 3.65, failing WCAG AA. This change
brings contrast to 4.50, passing WCAG AA.

PR-URL: #35063
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

doc/api_assets/hljs.css

@@ -15,7 +15,7 @@
 .hljs-string,
 .hljs-regexp,
 .hljs-number {
-  color: #E54305;
+  color: #cf350d;
 }
 
 .hljs-doctag {