Update README.md

Author: alesanchezr

exercises/03.1-Access-and-retrieve/README.md

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+---
+tutorial: https://www.youtube.com/watch?v=9-yAzjsWXtU
+---
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 # `03.1` Access and Retrieve
 
 Arrays are part of every programming language. They are the way to go when you want to have a "list of elements."
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 ```txt
 ⚠️ Important note: Use null as a value and not "null" as a string.
-```
+```