Create milesToKilometers.md

A simple but useful conversion snippet.

snippets/milesToKilometers.md

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+---
+title: milesToKilometers
+tags: math,beginner
+---
+
+Converts Miles to Kilometers.
+
+- Follows the formula `km = mi x 1.609344`.
+
+```js
+const milesToKilometers = distance => distance * 1.609344;
+```
+
+```js
+milesToKilometers(5); // 8.04672
+```