fix(dogfood): rename mux module input add-project to add_project (#22459)

The mux module's input variable was renamed from `add-project` to
`add_project`. This updates the dogfood template to use the new name.

Ref:
https://github.com/coder/registry/blob/main/registry/coder/modules/mux/main.tf
(variable `add_project`)

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

dogfood/coder/main.tf

@@ -379,7 +379,7 @@ module "mux" {
   agent_id        = coder_agent.dev.id
   subdomain       = true
   display_name    = "Mux"
-  add-project     = local.repo_dir
+  add_project     = local.repo_dir
   install_version = "next"
   package_manager = "bun"
 }