docs: correct Revolt config override filename

[ELginas]

Tested myself Revolt.override.toml doesn't work and Revolt.overrides.toml does indeed work and source code also confirms it:

backend/crates/core/config/src/lib.rs

Line 57 in a1b0e47

"Revolt.overrides.toml",

Please make sure to check the following tasks before opening and submitting a PR

• I understand and have followed the contribution guide
• I have tested my changes locally and they are working as intended

[insertish]

thanks for correction