Replace 'rackup' with 'rack' in server file

Running this example with Ruby 3.4.5 the following does not work:

```ruby
Rackup::Handler.get("puma").run(rack_app, Port: 9393, Host: "0.0.0.0")
```
It returns the following error message:
`uninitialized constant Rackup::Handler (NameError)`

To resolve this I just required Rack and changed Rackup to Rack.  Then things worked fine.

Author: joerzepiejewski

examples/streamable_http_server.rb

@@ -2,7 +2,7 @@
 
 $LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 require "mcp"
-require "rackup"
+require "rack"
 require "json"
 require "logger"
 
@@ -168,4 +168,4 @@ def call(message:, delay: 0)
 MESSAGE
 
 # Start the server
-Rackup::Handler.get("puma").run(rack_app, Port: 9393, Host: "localhost")
+Rack::Handler.get("puma").run(rack_app, Port: 9393, Host: "localhost")