Fix typos

s/propogate/propagate/g

Spotted while researching sendgrid-nodejs issues.

sendgrid/helpers/inbound/README.md

@@ -78,7 +78,7 @@ Update your SendGrid Incoming Parse settings: [Settings Page](https://app.sendgr
 
 Next, send an email to [anything]@inbound.yourdomain.com, then look at your Heroku logs: https://dashboard.heroku.com/apps/[name-of-your-app]/logs
 
-While you are waiting for your MX records to propogate, you can test by using the test data sender:
+While you are waiting for your MX records to propagate, you can test by using the test data sender:
 
 ```bash
 git clone https://github.com/sendgrid/sendgrid-python.git
@@ -115,7 +115,7 @@ This module parses the incoming POST data from a [Flask request object](http://f
 
 ## send.py & /sample_data
 
-This module is used to send sample test data. It is useful for testing and development, particularly while you wait for your MX records to propogate.
+This module is used to send sample test data. It is useful for testing and development, particularly while you wait for your MX records to propagate.
 
 <a name="testing"></a>
 # Testing the Source Code