Update describe.py
This PR fixes an issue where passing a single value to .describe(percentiles=[...]) would incorrectly include the 50th percentile (median) in the output. The fix ensures that only the explicitly requested percentiles are returned.

Changes:
Modified the .describe() method to avoid adding the 50th percentile by default when a custom percentiles list is provided.

Closes: #60550
Thanks for the PR, but https://github.com/pandas-dev/pandas/pull/61158 has addressed and closed the original issue so closing this PR. Happy to have your contributions towards other PRs