Fix incorrectly documented attribute in csv docs (GH-101250)

(cherry picked from commit 36b6709)

Co-authored-by: Reza Rastak <rezarastak@users.noreply.github.com>

Author: miss-islington

Doc/library/csv.rst

@@ -450,7 +450,7 @@ Reader objects have the following public attributes:
 
 DictReader objects have the following public attribute:
 
-.. attribute:: csvreader.fieldnames
+.. attribute:: DictReader.fieldnames
 
    If not passed as a parameter when creating the object, this attribute is
    initialized upon first access or when the first record is read from the