Align swagger schema get output.

The swagger object for schema get indicates an object name of
"schema_json" whilst the actual returned json in the body is "schema".
Changing to have swagger object called "schema" to be consistent
since this is what existing clients must have been forcing to for it to
work.

aries_cloudagent/messaging/schemas/routes.py

@@ -66,7 +66,7 @@ class SchemaSchema(OpenAPISchema):
 class SchemaGetResultsSchema(OpenAPISchema):
     """Results schema for schema get request."""
 
-    schema_json = fields.Nested(SchemaSchema())
+    schema = fields.Nested(SchemaSchema())
 
 
 class SchemasCreatedResultsSchema(OpenAPISchema):