Rename samples subdirectories

samples/Opa/hello_syntax1.opa

@@ -0,0 +1,9 @@
+/**
+ * To compile & run on port 8080:
+ * opa hello_syntax1.opa --
+ */
+server = Server.one_page_server(
+   "Hello, world",
+   -> (<h1>Hello, world</h1>)
+)
+

samples/Opa/hello_syntax2.opa

@@ -0,0 +1,11 @@
+/**
+ * To compile & run on port 8080:
+ * opa --parser js-like hello_syntax2.opa --
+ */
+Server.start(
+   Server.http,
+   {
+     page: function() { <h1>Hello, world</h1> },
+     title: "Hello, world"
+   }
+)