Fix build error: \filename{/dev/urandom} -> \file{/dev/urandom}.

Doc/whatsnew/whatsnew24.tex

@@ -1121,7 +1121,7 @@ be replaced by the option's default value.
 \item A new \function{urandom(\var{n})} function 
 was added to the \module{os} module, providing access to
 platform-specific sources of randomness such as
-\filename{/dev/urandom} on Linux or the Windows CryptoAPI.  The
+\file{/dev/urandom} on Linux or the Windows CryptoAPI.  The
 function returns a string containing \var{n} bytes of random data.
 (Contributed by Trevor Perrin.)