[fs.class.path.general] Defuse cross-reference to POSIX

Fixes ISO/CS comment (C++23 proof)

Author: jensmaurer

source/iostreams.tex

@@ -13595,7 +13595,8 @@
 a pathname to a particular file in a file hierarchy. There may be multiple
 pathnames that resolve to the same file.
 \begin{example}
-POSIX specifies the mechanism in section 4.12, Pathname resolution.
+For POSIX-based operating systems,
+this mechanism is specified in POSIX, section 4.12.
 \end{example}
 
 \begin{codeblock}