pythongh-137952: update csv.Sniffer().has_header() docs to describe the actual off-by-onish behavior (pythonGH-137953)

* checks 21, not 20
* Say "header" instead of "first row" to disambiguate per review.

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Co-authored-by: Stan Ulbrych <[email protected]>
Co-authored-by: Maurycy Pawłowski-Wieroński <[email protected]>

Author: maurycy

Doc/library/csv.rst

@@ -295,8 +295,8 @@ The :mod:`csv` module defines the following classes:
       - the second through n-th rows contain strings where at least one value's
         length differs from that of the putative header of that column.
 
-      Twenty rows after the first row are sampled; if more than half of columns +
-      rows meet the criteria, :const:`True` is returned.
+      Twenty-one rows after the header are sampled; if more than half of the
+      columns + rows meet the criteria, :const:`True` is returned.
 
    .. note::