LaTeX: fix a 7.4.0 typo in a default for \sphinxboxsetup

\sphinxbox and \sphinxboxsetup were added at 6.2.0. They are not used in
the LaTeX produced by Sphinx (nor are they tested by CI), they are only
provided for benefits of user redefinitions of default styling macros
for text, as a utility for decorative inline boxes.  At 7.4.0 the
default value for border-radius key which was \fboxsep got modified into
an explicit length which should have been 3pt but was written 0.4pt by
mistake.

sphinx/texinputs/sphinx.sty

@@ -9,7 +9,7 @@
 % by the Sphinx LaTeX writer.
 
 \NeedsTeXFormat{LaTeX2e}[1995/12/01]
-\ProvidesPackage{sphinx}[2024/10/11 v8.1.1 Sphinx LaTeX package (sphinx-doc)]
+\ProvidesPackage{sphinx}[2024/11/23 v8.1.3 Sphinx LaTeX package (sphinx-doc)]
 
 % provides \ltx@ifundefined
 % (many packages load ltxcmds: graphicx does for pdftex and lualatex but
@@ -1098,12 +1098,13 @@
 % Some of these defaults got already set.  But we now list them all explicitly
 % for a complete initial configuration of reset storage.
 % At 7.4.0, \fboxrule and \fboxsep replaced by 0.4pt and 3pt which are anyhow
-% the defaults for these LaTeX dimensions.
+% the defaults for these LaTeX dimensions. 8.1.3 corrected border-radius
+% default back to 3pt (\fboxsep) not 0.4pt (\fboxrule).
 \let\spx@boxes@sphinxbox@defaults\@gobble
 \sphinxboxsetup{%
                 border-width=0.4pt,
                 padding=3pt,
-                border-radius=0.4pt,
+                border-radius=3pt,
                 box-shadow=none,
 % MEMO: as xcolor is loaded, \spx@defineorletcolor has a "\colorlet" branch
 % which makes this syntax acceptable and avoids duplicating here the values.