Fix Slate template

Addresses PR #338

Author: terry90

templates/rspec_api_documentation/slate_example.mustache

@@ -9,7 +9,7 @@
 #### Endpoint
 
 {{# requests}}
-```
+```plaintext
 {{ request_method }} {{ request_path }}
 {{ request_headers_text }}
 ```
@@ -50,7 +50,7 @@ None known.
 
 ### Response
 
-```
+```plaintext
 {{ response_headers_text }}
 {{ response_status }} {{ response_status_text}}
 ```