Require type keyword in if/then/else example; fix $uses key name

[clemensv]

• Adds an explicit type: object keyword to the if/then/else example schemas, since (like allOf/anyOf/oneOf/not) each is itself a JSON Structure Core schema object and properties alone does not constrain the node to object.
• Adds a clarifying paragraph stating this requirement explicitly.
• Fixes the $uses extension key name in the enabling-the-extensions section and example from JSONSchemaConditionalComposition to the correct JSONStructureConditionalComposition.