[3.12] Doc: Fix default `latex_elements['papersize']` (GH-124525) (#124726)

Doc: Fix default ``latex_elements['papersize']`` (GH-124525)

https://www.sphinx-doc.org/en/master/latex.htmlGH-the-latex-elements-configuration-setting

It should be 'letterpaper' or 'a4paper' not 'letter' or 'a4'
(not to be confused with PAPER env variable).
(cherry picked from commit fae5058ec1)

Co-authored-by: Jean-François B <2589111+jfbu@users.noreply.github.com>

Doc/conf.py

@@ -391,8 +391,8 @@
 \let\endVerbatim=\endOriginalVerbatim
 \setcounter{tocdepth}{2}
 ''',
-    # The paper size ('letter' or 'a4').
-    'papersize': 'a4',
+    # The paper size ('letterpaper' or 'a4paper').
+    'papersize': 'a4paper',
     # The font size ('10pt', '11pt' or '12pt').
     'pointsize': '10pt',
 }