Follow upstream convention in shebang

Our fork uses '#!/perl (Editor hint)' but upstream normally uses '#!perl'.  Conforming to upstream.

FWIW, the Kate editor recognizes the former as perl for the purpose of syntax highlighting but does not recognize the latter. But the one guy I know who uses Kate didn't take the opportunity to object :)

plugins/user_config

@@ -1,4 +1,4 @@
-#!/perl (Editor hint)
+#!perl
 use strict;
 use warnings;