Update DEFAULT_SCHEMA_CLASSES default value in Settings docs (encode#7014)

The default value was changed to point to the OpenAPI AutoSchema class.  The docs were leading users to believe that rest_framework.schemas.AutoSchema was the default.  As of this commit, the root AutoSchema is in fact imported from the coreapi module.

Author: ajyong

docs/api-guide/settings.md

@@ -101,7 +101,7 @@ Default: `'rest_framework.negotiation.DefaultContentNegotiation'`
 
 A view inspector class that will be used for schema generation.
 
-Default: `'rest_framework.schemas.AutoSchema'`
+Default: `'rest_framework.schemas.openapi.AutoSchema'`
 
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