Update schema generator example (#4267)

1. The returns from the views needs to be a Response
2. I found that instantiating the generator at the module level caused an import error when starting Django, likely because it is executing before all the app load magic happened. Moving it into the view method solved this.

docs/api-guide/schemas.md

@@ -157,14 +157,14 @@ return the schema.
 **views.py:**
 
     from rest_framework.decorators import api_view, renderer_classes
-    from rest_framework import renderers, schemas
+    from rest_framework import renderers, response, schemas
 
-    generator = schemas.SchemaGenerator(title='Bookings API')
 
     @api_view()
     @renderer_classes([renderers.CoreJSONRenderer])
     def schema_view(request):
-        return generator.get_schema()
+        generator = schemas.SchemaGenerator(title='Bookings API')
+        return response.Response(generator.get_schema())
 
 **urls.py:**
 
@@ -185,7 +185,8 @@ you need to pass the `request` argument to the `get_schema()` method, like so:
     @api_view()
     @renderer_classes([renderers.CoreJSONRenderer])
     def schema_view(request):
-        return generator.get_schema(request=request)
+        generator = schemas.SchemaGenerator(title='Bookings API')
+        return response.Response(generator.get_schema(request=request))
 
 ## Explicit schema definition
 
@@ -196,7 +197,7 @@ representation.
 
     import coreapi
     from rest_framework.decorators import api_view, renderer_classes
-    from rest_framework import renderers
+    from rest_framework import renderers, response
 
     schema = coreapi.Document(
         title='Bookings API',
@@ -208,7 +209,7 @@ representation.
     @api_view()
     @renderer_classes([renderers.CoreJSONRenderer])
     def schema_view(request):
-        return schema
+        return response.Response(schema)
 
 ## Static schema file
 
@@ -273,7 +274,8 @@ Returns a `coreapi.Document` instance that represents the API schema.
     @api_view
     @renderer_classes([renderers.CoreJSONRenderer])
     def schema_view(request):
-        return generator.get_schema()
+        generator = schemas.SchemaGenerator(title='Bookings API')
+        return Response(generator.get_schema())
 
 Arguments: