README: fix signup link (#11)

README.md

@@ -9,7 +9,7 @@ This module contains the protobuf definitions for the Dispatch SDK.
 [connectrpc]:      https://connectrpc.com/
 [grpc]:            https://grpc.io/
 [http-signatures]: https://datatracker.ietf.org/doc/draft-ietf-httpbis-message-signatures/19/
-[signup]:          https://docs.stealthrocket.cloud/dispatch/getting-started
+[signup]:          https://docs.stealthrocket.cloud/stateful-functions/getting-started
 [rpc-dispatch]:    https://buf.build/stealthrocket/dispatch-sdk/docs/main:dispatch.sdk.v1#dispatch.sdk.v1.DispatchService.Dispatch
 [rpc-function]:    https://buf.build/stealthrocket/dispatch-sdk/docs/main:dispatch.sdk.v1#dispatch.sdk.v1.FunctionService.Run