[3.2.x] Refs #33182 -- Adjusted custom admin theming example to use c…

…orrect template block.

Backport of a754b82 from main

docs/ref/contrib/admin/index.txt

@@ -2869,7 +2869,7 @@ override to your project:
 
     {% extends 'admin/base.html' %}
 
-    {% block extrahead %}{{ block.super }}
+    {% block extrastyle %}{{ block.super }}
     <style>
     :root {
       --primary: #9774d5;