python3.pkgs.cmd2: fix shebang

The bang was missing!  What was there before would still have worked,
but I think it would end up being executed by /bin/sh since it had no
shebang line.

pkgs/development/python-modules/cmd2/default.nix

@@ -18,8 +18,8 @@ buildPythonPackage rec {
   postPatch = stdenv.lib.optional stdenv.isDarwin ''
     # Fake the impure dependencies pbpaste and pbcopy
     mkdir bin
-    echo '#${stdenv.shell}' > bin/pbpaste
-    echo '#${stdenv.shell}' > bin/pbcopy
+    echo '#!${stdenv.shell}' > bin/pbpaste
+    echo '#!${stdenv.shell}' > bin/pbcopy
     chmod +x bin/{pbcopy,pbpaste}
     export PATH=$(realpath bin):$PATH
   '';