zsh:pdfunite(): Fix execution through missing $0

I don't know what happened there. Apparently I just checked that the
early-abort condition works.

.config/zsh/zshrc.d/40-functions.zsh

@@ -628,7 +628,7 @@ pdfunite() {
 		return 2
 	fi
 
-	command "$@"
+	command pdfunite "$@"
 }
 
 # List pids of processes that use an open file