DOC: Improve clarity of GroupBy introduction sentence

This small change clarifies the introductory sentence of the GroupBy user guide, as recommended for documentation improvements. It makes the definition of the "group by" process more direct and easier for new users to understand.

Author: Vedant-Kadam-Noobie

doc/source/user_guide/groupby.rst

@@ -6,8 +6,7 @@
 Group by: split-apply-combine
 *****************************
 
-By "group by" we are referring to a process involving one or more of the following
-steps:
+A "group by" operation involves some combination of the following steps:
 
 * **Splitting** the data into groups based on some criteria.
 * **Applying** a function to each group independently.