gnuplot: use gnuplot license

pkgs/tools/graphics/gnuplot/default.nix

@@ -123,15 +123,7 @@ in
     homepage = "http://www.gnuplot.info/";
     description = "Portable command-line driven graphing utility for many platforms";
     platforms = platforms.linux ++ platforms.darwin;
-    license = {
-      # Essentially a BSD license with one modification:
-      # Permission to modify the software is granted, but not the right to
-      # distribute the complete modified source code.  Modifications are to
-      # be distributed as patches to the released version.  Permission to
-      # distribute binaries produced by compiling modified sources is granted,
-      # provided you: ...
-      url = "https://sourceforge.net/p/gnuplot/gnuplot-main/ci/master/tree/Copyright";
-    };
+    license = lib.licenses.gnuplot;
     maintainers = with maintainers; [ lovek323 ];
     mainProgram = "gnuplot";
   };