fix docker link on install page (#964) (#972)

Author: lunny

templates/install.tmpl

@@ -8,7 +8,7 @@
 			<div class="ui attached segment">
 				{{template "base/alert" .}}
 
-				<p>{{.i18n.Tr "install.docker_helper" "https://github.com/go-gitea/gitea/tree/master/docker" | Safe}}</p>
+				<p>{{.i18n.Tr "install.docker_helper" "https://docs.gitea.io/en-us/install-with-docker/" | Safe}}</p>
 
 				<form class="ui form" action="{{AppSubUrl}}/install" method="post">
 					<!-- Dtabase Settings -->