software/html5as: document that download_url parameter is actually optional

software/html5as/README.md

@@ -6,9 +6,9 @@
 
 * Use Nginx server
 
-## Parameter ##
+## Parameters ##
 
-download_url (string) :required
+`download_url` (string): the URL of an archive to download and use as document root.
 
 Details :
 
@@ -20,4 +20,4 @@ Details :
 
 ## How it works ##
 
-Each time you (re)start your instance or update parameters, html5as will remove previous website then download tarball and extract it in docroot directory.
+When `download_url` parameter is provided, each time you (re)start your instance or update parameters, html5as will remove previous website then download tarball and extract it in document root directory.