Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -335,6 +335,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | May 18, 2025 | [1931](https://leetcode.com/problems/painting-a-grid-with-three-different-colors/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1931) | 🔴 Hard |
 | May 19, 2025 | [3024](https://leetcode.com/problems/type-of-triangle/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3024) | 🟢 Easy |
 | May 20, 2025 | [3355](https://leetcode.com/problems/zero-array-transformation-i/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3355) | 🟡 Medium |
+| May 21, 2025 | [73](https://leetcode.com/problems/set-matrix-zeroes/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/73) | 🟡 Medium |
 
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