terraformls: Fix arguments to cmd

```console
$ terraform-ls
Usage: terraform-ls [--version] [--help] <command> [<args>]

Available commands are:
    completion        Lists available completion items
    inspect-module    Lists available debug items
    serve             Starts the Language Server
```

```console
$ terraform-ls serve
2020/09/14 13:34:28 serve_command.go:137: Starting terraform-ls 0.0.0-dev
2020/09/14 13:34:28 service.go:60: Preparing new session ...
2020/09/14 13:34:28 service.go:77: Worker pool size set to 12
2020/09/14 13:34:28 watcher.go:118: watching for changes ...
2020/09/14 13:34:28 langserver.go:69: Starting server (pid 83736) ...
```

1 files changed, 1 insertions, 1 deletions

diff --git a/lua/lspconfig/terraformls.lua b/lua/lspconfig/terraformls.lua
index 7d551e5c..90c973dd 100644
--- a/lua/lspconfig/terraformls.lua
+++ b/lua/lspconfig/terraformls.lua
@@ -3,7 +3,7 @@ local util = require 'lspconfig/util'
 
 configs.terraformls = {
   default_config = {
-    cmd = {"terraform-ls"};
+    cmd = {"terraform-ls", "serve"};
     filetypes = {"terraform"};
     root_dir = util.root_pattern(".terraform", ".git");
   };