Fix docu anchors

Author: jochenehret

docs/v3/source/includes/resources/spaces/_get_usage_summary.md.erb

@@ -1,4 +1,4 @@
-### Get usage summary
+### Get space usage summary
 
 ```
 Example Request

docs/v3/source/includes/upgrade_guide/conceptual_changes/_resource_summaries.md

@@ -71,5 +71,5 @@ in the response body.
 #### Usage summary endpoints
 
 There are still a couple of endpoints in V3 that provide a basic summary of
-instance and memory usage. See the [org summary](#get-usage-summary) and
+instance and memory usage. See the [org summary](#get-usage-summary), [space summary](#get-space-usage-summary) and
 [platform summary](#get-platform-usage-summary) endpoints.