Add configuration doc for keep_legacy_inflector

Author: jonnyeom

core/configuration.md

@@ -68,6 +68,9 @@ api_platform:
     # Enable the data collector and the WebProfilerBundle integration.
     enable_profiler: true
 
+    # Keep doctrine/inflector instead of symfony/string to generate plurals for routes.
+    keep_legacy_inflector: true
+
     collection:
         # The name of the query parameter to filter nullable results (with the ExistsFilter).
         exists_parameter_name: 'exists'