Fix command name in cargo nih-plug usage string

cargo_nih_plug/src/main.rs

@@ -2,5 +2,5 @@ fn main() -> nih_plug_xtask::Result<()> {
     // This includes both the `cargo` command and the `nih-plug` subcommand, so we should get rid of
     // those first
     let args = std::env::args().skip(2);
-    nih_plug_xtask::main_with_args(args)
+    nih_plug_xtask::main_with_args("cargo nih-plug", args)
 }