Minor imporvements to terraform ignore file

* Fix Typo
* Add example to include overrides

Author: alastairtree

Terraform.gitignore

@@ -14,8 +14,13 @@ crash.log
 #
 # example.tfvars
 
-# Ignore override files as they are usually used to override ressources locally
+# Ignore override files as they are usually used to override resources locally and so
+# are not checked in
 override.tf
 override.tf.json
 *_override.tf
 *_override.tf.json
+
+# Include override files you do wish to add to version control using negated pattern
+#
+# !example_override.tf