Update README.md

Author: shahariaazam

README.md

@@ -126,7 +126,7 @@ $subscription->intervalLength          = "1";
 $subscription->intervalUnit            = "months";
 $subscription->startDate               = "2011-03-12";
 $subscription->totalOccurrences        = "12";
-$subscription->amount                  = "12.99");
+$subscription->amount                  = "12.99";
 $subscription->creditCardCardNumber    = "6011000000000012";
 $subscription->creditCardExpirationDate= "2018-10";
 $subscription->creditCardCardCode      = "123";