[release-notes] index: reformat performance mode note

[Link Dupont <link src gnome org>]

commit 2d67f6a4881e1dc98299b0ea594646f718ca1baf
Author: Link Dupont <link sub-pop net>
Date:   Wed Sep 8 21:32:02 2021 -0400

    index: reformat performance mode note

 help/C/index.page | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)
---
diff --git a/help/C/index.page b/help/C/index.page
index 76093fce..e350f4b5 100644
--- a/help/C/index.page
+++ b/help/C/index.page
@@ -72,9 +72,11 @@ xmlns:e="http://projectmallard.org/experimental/"; type="guide" id="index">
       consumption.</p></item>
     </list>
     
-    <p>Note: performance mode is only available on hardware that supports this 
-    feature. On hardware that doesn't have performance mode, the performance
-    mode option is not shown.</p>
+    <note style="tip">
+      <p>Performance mode is only available on hardware that supports this 
+      feature. On hardware that doesn't have performance mode, the performance
+      mode option is not shown.</p>
+    </note>
     
     <p>With GNOME 41, the power mode feature has been improved. The mode can 
     now be quickly changed from the <gui>System Status</gui> menu.