Fix example in README
Fix example in README

The example fails to run because "net/mail" is imported and not used.
VividCortex/sendgrid-go,VividCortex/sendgrid-go,sendgrid/sendgrid-go,karmakaze/sendgrid-go,sendgrid/sendgrid-go,ChannelMeter/sendgrid-go,karmakaze/sendgrid-go