[Resolves Sceptre#605] Fix documentation mismatch regarding protected…

… Stacks

- Now the documentation is aligned with the implementation and specifies that the
  word "protect" should be used to protect stacks (instead of "protected")

docs/docs/stack_config.md

@@ -20,7 +20,7 @@ a particular Stack. The available keys are listed below.
 * [notifications](#notifications) _(optional)_
 * [on_failure](#on_failure) _(optional)_
 * [parameters](#parameters) _(optional)_
-* [protected](#protected) _(optional)_
+* [protect](#protect) _(optional)_
 * [role_arn](#role_arn) _(optional)_
 * [sceptre_user_data](#sceptre_user_data) _(optional)_
 * [stack_name](#stack_name) _(optional)_
@@ -119,7 +119,7 @@ parameters:
     - !file_contents /file/with/security_group_id.txt
 ```
 
-### protected
+### protect
 
 Stack protection against execution of the following commands:
 
@@ -132,6 +132,12 @@ Stack protection against execution of the following commands:
 If a user tries to run one of these commands on a protected Stack, Sceptre will
 throw an error.
 
+Example:
+
+```yaml
+protect: true
+```
+
 ### role_arn
 
 The ARN of a [CloudFormation Service