Fix typo

Author: romain-at-adacore

courses/rust_essentials/050_tuples_and_arrays/99_summary.rst

@@ -15,7 +15,7 @@ What We Covered
 
   **Tuples**
   - Group values of different types, fixed size
-  - Fields are accessed via dot notation followed by its index
+  - Fields are accessed via dot notation followed by their index
     - Starting from :rust:`0`
 
 - **Patterns and Destructuring**