man-pages-posix: use version format as repology

repology considers 2017a, not 2017-a as current since all other
distributions seem to use that versioning scheme.

pkgs/data/documentation/man-pages-posix/default.nix

@@ -1,11 +1,16 @@
 {lib, stdenv, fetchurl}:
 
+let
+  year = "2017";
+  minor = "a";
+in
+
 stdenv.mkDerivation rec {
   pname = "man-pages-posix";
-  version = "2017-a";
+  version = "${year}${minor}";
 
   src = fetchurl {
-    url = "mirror://kernel/linux/docs/man-pages/man-pages-posix/${pname}-${version}.tar.xz";
+    url = "mirror://kernel/linux/docs/man-pages/man-pages-posix/${pname}-${year}-${minor}.tar.xz";
     sha256 = "ce67bb25b5048b20dad772e405a83f4bc70faf051afa289361c81f9660318bc3";
   };