Use parentheses in GRPC.Stub to avoid warnings during compilation in …

[davidwebster48]

It resolves this warning reported during compilation:

warning: using map.field notation (without parentheses) to invoke function Rpc.LandingPagesManager.V1.RPCService.Service.__rpc_calls__() is deprecated, you must add parentheses instead: remote.function()
  lib/rpc/landing_pages_manager/v1/landing_pages_manager_service.pb.ex:56: (module)
  (elixir 1.17.3) src/elixir_compiler.erl:77: :elixir_compiler.dispatch/4
  (elixir 1.17.3) src/elixir_compiler.erl:52: :elixir_compiler.compile/4
  (elixir 1.17.3) src/elixir_module.erl:427: :elixir_module.eval_form/7
  (elixir 1.17.3) src/elixir_module.erl:129: :elixir_module.compile/7

This PR applies the same fix as #16 but in a different module.