chore: update main.tf with file("example.private-key.pem")

[Crash-GHaun]

Just updating so if someone isn't following a guide it's easy for them to insert their pem file

[apeabody]

/gcbrun

[apeabody]

Thanks for the contribution @Crash-GHaun!

For this change to work, please also include the sample file:

terraform_validate ./examples/gh-runner-gke-simple
╷
│ Error: Invalid function argument
│ 
│   on main.tf line 26, in module "runner-gke":
│   26:   gh_app_private_key     = file("example.private-key.pem")
│     ├────────────────
│     │ while calling file(path)
│ 
│ Invalid value for "path" parameter: no file exists at
│ "example.private-key.pem"; this function works only with files that are
│ distributed as part of the configuration source code, so if this file will
│ be created by a resource in this configuration you must instead obtain this
│ result from an attribute of that resource.