DOC: Standardize use of "Iterate" and "Iterator" (pandas-dev#27871)

changed `Iterator over` --> `Iterate over`

closes pandas-dev#27861

Author: sameshl

pandas/core/frame.py

@@ -770,7 +770,7 @@ def style(self):
     _shared_docs[
         "items"
     ] = r"""
-        Iterator over (column name, Series) pairs.
+        Iterate over (column name, Series) pairs.
 
         Iterates over the DataFrame columns, returning a tuple with
         the column name and the content as a Series.