Update styleguide.md
Update styleguide.md

Made a change to SMTP and added X-SMTPAPI. This is to break up content and make it easier to find what you are looking for. This is linked to https://github.com/sendgrid/docs/pull/4248
ksigler7/docs,ksigler7/docs,Whatthefoxsays/docs,Whatthefoxsays/docs,ksigler7/docs,Whatthefoxsays/docs,sendgrid/docs,sendgrid/docs,sendgrid/docs