[intro.refs] Update dated reference to previous POSIX standard

jwakely · tkoeppe · commit 1567c481e3ca · 2021-11-09T09:53:04.000Z
Fixes #5098
diff --git a/source/intro.tex b/source/intro.tex
@@ -92,7 +92,7 @@
 \end{footnote}
 
 \pnum
-The operating system interface described in ISO/IEC 9945:2003 is
+The operating system interface described in ISO/IEC 9945:2009 is
 hereinafter called \defn{POSIX}.
 
 \pnum
