Tweak Security settings section headers

This brings us slightly closer to the version Design is working from in terms of
section naming.

Prep for https://github.com/vector-im/element-web/issues/13895

res/css/views/settings/tabs/_SettingsTab.scss

@@ -1,5 +1,5 @@
 /*
-Copyright 2019 New Vector Ltd
+Copyright 2019, 2020 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@ limitations under the License.
     font-size: $font-20px;
     font-weight: 600;
     color: $primary-fg-color;
+    margin-bottom: 10px;
 }
 
 .mx_SettingsTab_heading:nth-child(n + 2) {