Document usdc_token in vesting create_vault examples

[silentgeckoaudit3801]

Fixes #357.

Updates vesting.md so the documented create_vault signature and usage examples match the current contract API:

• adds the leading usdc_token: Address parameter to the create_vault signature and parameter list
• documents the current Result<u32, Error> return type
• passes usdc_token in the create, release, redirect, and cancel examples that call token-moving entrypoints

Validation: static documentation/API review only; no project toolchain commands were run from this environment.

[1nonlypiece]

@silentgeckoaudit3801 this PR needs to be linked to an issue you're actually assigned to before it can be merged through the campaign — closing for now, happy to take another look once that's in place.