Fix indentation

Author: Austin-X

guides/type_definitions/directives.md

@@ -35,6 +35,7 @@ Here's how the two built-in directives work:
 
 - `@skip(if: ...)` skips the selection if the `if: ...` value is truthy ({{ "GraphQL::Schema::Directive::Skip" | api_doc }})
 - `@include(if: ...)` includes the selection if the `if: ...` value is truthy ({{ "GraphQL::Schema::Directive::Include" | api_doc }})
+
 ### Custom Runtime Directives
 
 Custom directives extend {{ "GraphQL::Schema::Directive" | api_doc }}: