Rename Closures/README.md to Closures/closures.md

Renamed the Closures README file to closures.md and updated the corresponding link in index.md to reflect the new filename.

Author: adityaSrivastava29

Closures/README.md renamed to Closures/closures.md

@@ -3,7 +3,7 @@
 Welcome! Use the links below to navigate the main topics:
 
 - [Functions](functions/functions.md)
-- [Closures](Closures/README.md)
+- [Closures](Closures/closures.md)
 - [Event Loop](JS%20Interview%20Question/eventloop.md)
 
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