DOC: Clarify 'it' in aggregate doc (pandas-dev#16989)

Closes pandas-devgh-16988.

Author: gfyoung

pandas/core/generic.py

@@ -3152,7 +3152,7 @@ def pipe(self, func, *args, **kwargs):
     (e.g., np.mean(arr_2d, axis=0)) as opposed to
     mimicking the default Numpy behavior (e.g., np.mean(arr_2d)).
 
-    agg is an alias for aggregate. Use it.
+    `agg` is an alias for `aggregate`. Use the alias.
 
     Returns
     -------