Fixed the memory usage explanation of categorical in gotchas from O(nm) to O(n+m) (#17736)

Author: berkay-dincer

doc/source/categorical.rst

@@ -979,7 +979,7 @@ Memory Usage
 
 .. _categorical.memory:
 
-The memory usage of a ``Categorical`` is proportional to the number of categories times the length of the data. In contrast,
+The memory usage of a ``Categorical`` is proportional to the number of categories plus the length of the data. In contrast,
 an ``object`` dtype is a constant times the length of the data.
 
 .. ipython:: python