Update DIRECTORY.md (#66)

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Author: github-actions[bot]

DIRECTORY.md

@@ -15,6 +15,7 @@
 - [Prime](math/prime.jule)
 - [Q Rsqrt](math/q_rsqrt.jule)
 - [Round](math/round.jule)
+- [Stddev](math/stddev.jule)
 - [Sum](math/sum.jule)
 
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