Fix header level for new section

The Structural Interoperability section should be a subsection
of the OpenAPI Description Structure section.

Author: handrews

versions/3.1.1.md

@@ -140,7 +140,7 @@ An OpenAPI Description (OAD) MAY be made up of a single document or be divided i
 
 It is RECOMMENDED that the entry OpenAPI document be named: `openapi.json` or `openapi.yaml`.
 
-### <a name="structuralInteroperability"></a>Structural Interoperability
+#### <a name="structuralInteroperability"></a>Structural Interoperability
 
 When parsing an OAD, JSON or YAML objects are parsed into specific Objects (such as [Operation Objects](#operationObject), [Response Objects](#responseObject), [Reference Objects](#referenceObject), etc.) based on the parsing context.  Depending on how references are arranged, a given JSON or YAML object can be parsed in multiple different contexts: