Updated readme.md
Updated readme.md

Added toString() conversion of destination number, otherwise SMS dialog closes itself immediately on iOS 10.3.3.
vkeepe/cordova-sms-plugin,cordova-sms/cordova-sms-plugin,cordova-sms/cordova-sms-plugin,cordova-sms/cordova-sms-plugin,vkeepe/cordova-sms-plugin,vkeepe/cordova-sms-plugin