docs: update template_file examples

Use the new non-deprecated style.

refs #4025

website/source/docs/configuration/interpolation.html.md

@@ -178,7 +178,7 @@ A template resource looks like:
 
 ```
 resource "template_file" "example" {
-    filename = "template.txt"
+  template = "${hello} ${world}!"
   vars {
     hello = "goodnight"
     world = "moon"
@@ -190,17 +190,10 @@ output "rendered" {
 }
 ```
 
-Assuming `template.txt` looks like this:
-
-```
-${hello} ${world}!
-```
-
 Then the rendered value would be `goodnight moon!`.
 
 You may use any of the built-in functions in your template.
 
-
 ### Using Templates with Count
 
 Here is an example that combines the capabilities of templates with the interpolation
@@ -221,7 +214,7 @@ variable "hostnames" {
 resource "template_file" "web_init" {
   // here we expand multiple template_files - the same number as we have instances
   count    = "${var.count}"
-  filename = "templates/web_init.tpl"
+  template = "${file("templates/web_init.tpl")}"
   vars {
     // that gives us access to use count.index to do the lookup
     hostname = "${lookup(var.hostnames, count.index)}"