Fix typo.

Author: jmdobry

functions/sendgrid/index.js

@@ -103,7 +103,7 @@ function getPayload (requestBody) {
  * https://[YOUR_REGION].[YOUR_PROJECT_ID].cloudfunctions.net/sendEmail?sg_key=[YOUR_API_KEY]
  *
  * @example
- * curl -X POST "https://us-central1.your-project-id.cloudfunctions.net/sendEmail?sg_key=your_api_key" --data '{"to":"bob@email.com","from":"[email protected]","subject":"Hello from Sendgrid!","body":"Hello World!"}'
+ * curl -X POST "https://us-central1.your-project-id.cloudfunctions.net/sendEmail?sg_key=your_api_key" --data '{"to":"bob@email.com","from":"[email protected]","subject":"Hello from Sendgrid!","body":"Hello World!"}' --header "Content-Type: application/json"
  *
  * @param {Object} req Cloud Function request context.
  * @param {Object} req.query The parsed querystring.