Fix typo in charter 2023 (#644)

admin/webappsec-charter-2023.html

@@ -582,7 +582,7 @@ <h3>
           </dd>
           <dt><a href="https://github.com/mikewest/securer-contexts">Securer Contexts</a></dt>
           <dd>
-            Securer context is a proposal to extend the thread model beyond encrypting
+            Securer context is a proposal to extend the threat model beyond encrypting
             the transport layer, and bring attention to application layer threats that
             rely on either injection or insufficient isolation.
           </dd>