Update functions.md

Author: sadanandpai

challenges/functions.md

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-  <a href="/README.md#javascript-modern-interview-code-challenges-by-topic" id="home">Home</a>
+  <a href="/README.md#this-is-a-collection-of-modern-interview-code-challenges-on-javascript-suitable-for" id="home">Home</a>
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 ## JavaScript interview code challenges on Functions