add missing closing brace to media dicts section

docs/02_return_values.rst

@@ -430,6 +430,7 @@ Media dicts
         'foreground': # Estimated foreground colour for the attachment thumbnail
         'background': # Estimated background colour for the attachment thumbnail
         'accent': # Estimated accent colour for the attachment thumbnail
+    }
 
 Card dicts
 ----------
@@ -780,4 +781,4 @@ Admin retention dicts
         'rate': # Fraction of users retained 
         'value': # Absolute number of users retained
     }
-    
+