Fix private attributes ignore in documentation (#6601)

Author: delimitry

docs/api-guide/serializers.md

@@ -965,7 +965,7 @@ The following class is an example of a generic serializer that can handle coerci
         def to_representation(self, obj):
             for attribute_name in dir(obj):
                 attribute = getattr(obj, attribute_name)
-                if attribute_name('_'):
+                if attribute_name.startswith('_'):
                     # Ignore private attributes.
                     pass
                 elif hasattr(attribute, '__call__'):

docs/community/3.0-announcement.md

@@ -523,7 +523,7 @@ The following class is an example of a generic serializer that can handle coerci
         def to_representation(self, obj):
             for attribute_name in dir(obj):
                 attribute = getattr(obj, attribute_name)
-                if attribute_name('_'):
+                if attribute_name.startswith('_'):
                     # Ignore private attributes.
                     pass
                 elif hasattr(attribute, '__call__'):