Change schedule to show PSF as Silver sponsor

[kjaymiller]

on leadership_summit_2024.md

10:15am - 10:20am: Gold Sponsor Lightning Talk - Python Software Foundation

Should be

10:15am - 10:20am: Silver Sponsor Lightning Talk - Python Software Foundation

[cmm25]

Hi Sir. Kindly assign me this issue

[cmm25]

@kjaymiller check the pull request #428