Update Readme

Author: HenryEffiong

README.md

@@ -69,7 +69,7 @@ _
         final Flutterwave flutterwave = Flutterwave(
 	        context: context, publicKey: "Public Key-here",
     		currency: "currency-here",   
-            redirectUrl: "add-your-redirect-url-here"   
+            redirectUrl: "add-your-redirect-url-here",  
             txRef: "add-your-unique-reference-here",   
             amount: "3000",   
             customer: customer,