fix: replace .english.md extension with .md

curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-15-lattice-paths.md

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+---
+id: 5900f37b1000cf542c50fe8e
+challengeType: 5
+title: 'Problem 15: Lattice paths'
+forumTopicId: 301780
+---
+
+## Description
+<section id='description'>
+
+Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.
+
+<img class="img-responsive center-block" alt="a diagram of 6 2 by 2 grids showing all the routes to the bottom right corner" src="https://cdn-media-1.freecodecamp.org/project-euler/1Atixoj.gif" style="background-color: white; padding: 10px;">
+
+How many such routes are there through a given `gridSize`?
+
+</section>
+
+## Instructions
+<section id='instructions'>
+
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>latticePaths(4)</code> should return a number.
+    testString: assert(typeof latticePaths(4) === 'number');
+  - text: <code>latticePaths(4)</code> should return 70.
+    testString: assert.strictEqual(latticePaths(4), 70);
+  - text: <code>latticePaths(9)</code> should return 48620.
+    testString: assert.strictEqual(latticePaths(9), 48620);
+  - text: <code>latticePaths(20)</code> should return 137846528820.
+    testString: assert.strictEqual(latticePaths(20), 137846528820);
+
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function latticePaths(gridSize) {
+
+  return true;
+}
+
+latticePaths(4);
+```
+
+</div>
+
+
+
+</section>
+
+## Solution
+<section id='solution'>
+
+
+```js
+function latticePaths(gridSize) {
+  let paths = 1;
+
+  for (let i = 0; i < gridSize; i++) {
+    paths *= (2 * gridSize) - i;
+    paths /= i + 1;
+  }
+  return paths;
+}
+```
+
+</section>