Update discord icon to have transparency (#4466)

- **PR Description**

Fixes https://github.com/jesseduffield/lazygit/issues/4464.

I checked, and the README already has a width specified on the `img`
tag, so I don't think the increased size of this will matter at all. I
just grabbed this directly from discord's site
https://discord.com/branding


f7787fc819/README.md (L510)

I personally use github dark mode all the time, and can vouch that it
looks totally fine. Light mode users won't see a difference, since we
just removed some white


![Image](https://github.com/user-attachments/assets/f682a58b-eaff-44ed-89aa-69f19d797557)
This commit is contained in:
Jesse Duffield 2025-04-08 23:08:27 +10:00 committed by GitHub
commit 1f156c0da2
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