FIX: fixes documentation issue that states that template id is a string

[deB4SH]

Hi, based on your api-reference an integer is required for sending mails with a template id. The documentation states a string here, which will result in a Bad-Request response from your api.

Reference: https://dev.mailjet.com/email/reference/send-emails/

This just simply fixes the documentation for this particular part. :)