Updated PEN to use the proper currency code.

Peru updated their currency's symbol for the sol (previously nuevo sol) back in 2015 from S/. to S/.
diff --git a/lib/src/intl/constants.dart b/lib/src/intl/constants.dart
index 83e2607..cab45d1 100644
--- a/lib/src/intl/constants.dart
+++ b/lib/src/intl/constants.dart
@@ -146,7 +146,7 @@
   'SOS': 'SOS',
   'TZS': 'TSh',
   'UGX': 'UGX',
-  'PEN': 'S/.',
+  'PEN': 'S/',
   'KGS': 'KGS',
   'UZS': 'so\u02bcm',
   'TJS': 'Som',