Fix typo in docstring of DataFrame.memory_usage (#25761)

Author: pjleimbigler

pandas/core/frame.py

@@ -2522,8 +2522,8 @@ def memory_usage(self, index=True, deep=False):
         ----------
         index : bool, default True
             Specifies whether to include the memory usage of the DataFrame's
-            index in returned Series. If ``index=True`` the memory usage of the
-            index the first item in the output.
+            index in returned Series. If ``index=True``, the memory usage of 
+            the index is the first item in the output.
         deep : bool, default False
             If True, introspect the data deeply by interrogating
             `object` dtypes for system-level memory consumption, and include