docs: mark kurtosis task complete

Author: pavanvamsi3

PRD.md

@@ -16,5 +16,5 @@ We are adding new statistical functions that follow the existing patterns in `ma
 - [x] Add `interquartileRange(arr)` — returns Q3 - Q1 (the 75th percentile minus the 25th percentile). Should reuse the percentile function internally. Add to `math_modules/interquartileRange.js`, export from `index.js`, add tests.
 - [x] Add `zScore(arr, value)` — returns how many standard deviations a value is from the mean: `(value - mean) / standardDeviation`. Should reuse existing mean and standardDeviation functions. Add to `math_modules/zScore.js`, export from `index.js`, add tests.
 - [x] Add `skewness(arr)` — returns the sample skewness (Fisher's definition). Positive = right tail, negative = left tail, 0 = symmetric. Add to `math_modules/skewness.js`, export from `index.js`, add tests with known symmetric and skewed datasets.
-- [ ] Add `kurtosis(arr)` — returns the excess kurtosis. Normal distribution = 0. Add to `math_modules/kurtosis.js`, export from `index.js`, add tests.
+- [x] Add `kurtosis(arr)` — returns the excess kurtosis. Normal distribution = 0. Add to `math_modules/kurtosis.js`, export from `index.js`, add tests.
 - [ ] Update `README.md` — add documentation for all six new functions with usage examples, following the existing format.