provider/triton: fixup key_material in docs to use file()

Closes hashicorp#6211

Author: phinze

website/source/docs/providers/triton/index.html.markdown

@@ -17,7 +17,7 @@ Use the navigation to the left to read about the available resources.
 ```
 provider "triton" {
     account      = "AccountName"
-    key_material = "~/.ssh/id_rsa"
+    key_material = "${file("~/.ssh/id_rsa")}"
     key_id       = "25:d4:a9:fe:ef:e6:c0:bf:b4:4b:4b:d4:a8:8f:01:0f"
 
     # If using a private installation of Triton, specify the URL