Update part3b.md, improved English

the -> their sounds better.

Author: popovidis

src/content/3/en/part3b.md

@@ -93,7 +93,7 @@ For a decade, [Heroku](http://heroku.com) was dominating the PaaS scene. Unfortu
 
 We are now introducing two services [Fly.io](https://fly.io/) and [Render](https://render.com/) that both have a (limited) free plan. Fly.io is our "official" hosting service since it can be for sure used also on the parts 11 and 13 of the course. Render will be fine at least for the other parts of this course.
 
-Note that despite using the free tier only, Fly.io <i>might</i> require one to enter the credit card details. At the moment Render can be used without a credit card.
+Note that despite using the free tier only, Fly.io <i>might</i> require one to enter their credit card details. At the moment Render can be used without a credit card.
 
 Render might be a bit easier to use since it does not require any software to be installed on your machine.
 
@@ -120,7 +120,7 @@ Now we are using the port defined in the [environment variable](https://en.wikip
 
 #### Fly.io
 
-<i>Note that you may need to give your credit card number to Fly.io even if you are using only the free tier!</i> There has been actually conflicting reports about this, it is known for a fact that some of the students in this course are using Fly.io without entering the credit card info. At the moment [Render](https://render.com/) can be used without a credit card.
+<i>Note that you may need to give your credit card number to Fly.io even if you are using only the free tier!</i> There has been actually conflicting reports about this, it is known for a fact that some of the students in this course are using Fly.io without entering their credit card info. At the moment [Render](https://render.com/) can be used without a credit card.
 
 By default, everyone gets two free virtual machines that can be used for running two apps at the same time.