fix: replace .english.md extension with .md

curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-407-idempotents.md

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+---
+id: 5900f5041000cf542c510016
+challengeType: 5
+title: 'Problem 407: Idempotents'
+forumTopicId: 302075
+---
+
+## Description
+<section id='description'>
+If we calculate a2 mod 6 for 0 ≤ a ≤ 5 we get: 0,1,4,3,4,1.
+
+
+The largest value of a such that a2 ≡ a mod 6 is 4.
+Let's call M(n) the largest value of a < n such that a2 ≡ a (mod n).
+So M(6) = 4.
+
+
+Find ∑M(n) for 1 ≤ n ≤ 107.
+</section>
+
+## Instructions
+<section id='instructions'>
+
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>euler407()</code> should return 39782849136421.
+    testString: assert.strictEqual(euler407(), 39782849136421);
+
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function euler407() {
+
+  return true;
+}
+
+euler407();
+```
+
+</div>
+
+
+
+</section>
+
+## Solution
+<section id='solution'>
+
+```js
+// solution required
+```
+
+</section>