docs: update links to old docs

app/views/help/outgoing.html.haml

@@ -57,4 +57,4 @@
   .u-margin
     %h2.pageContent__subTitle Sending over HTTP using our API
     %p.pageContent__text
-      For full information about how to use our HTTP API, please #{link_to 'see the documentation', 'https://github.com/atech/postal/wiki/Using-the-API', :class => "u-link"}.
+      For full information about how to use our HTTP API, please #{link_to 'see the documentation', 'https://docs.postalserver.io/developer/api', :class => "u-link"}.